I am going to give this a shot; I have no real dog in the fight, since I'm not even sure what CL shenanigans the Ur-Priest build is using, and I don't care. All I care about is getting consistent definitions, and Frank's definition is bullshit.
Okay:
"+1 level of existing class". This is on numerous charts, and it always means "advance spells known and spells per day, but nothing else". It sometimes says "existing arcane spellcasting class" or "existing divine spellcasting class". I think this is pretty clearly adding "levels in a spellcasting class", because it literally says that in many cases.
To go along with this, the PrC descriptions always use some variant of "whenever a new (Derpmaster) level is gained, the character gains new spells per day as if she had also gained a level in whatever spellcasting class she belonged to before she added the prestige class".
In Complete Divine, at least, it clarifies this by saying "This essentially means that she adds the level of (Derpmaster) to the level of whatever other spellcasting class the character has, then determines spells per day accordingly".
It also usually clarifies that a.) you don't get any other benefits from raising a level in that (prior) class, and b.) if you have more than one spellcasting class, you pick one to advance (not all of them).
Contrasting this...things that give you "+1 caster level" are always understood to NOT advance spells per day and/or spells known; they don't advance anything, they are simply a numeric adder for making calculations based on caster level. Practiced spellcaster, domains, ioune stones...all of these add to caster level ONLY.
I don't see how caster level could possibly be synonomous with "levels in a spellcasting class". PrCs clearly advance your "levels in a spellcasting class"...which means your class level ONLY for the purpose of determining spells per day, spells known, and caster level (which is a separate thing). But when you advance your Paladin casting with a PrC, it adds 1 to your paladin level for the purposes of spellcasting...that is one "level in a spellcasting class", and it may or may not give you any new spells and it may or may not increase your CL, depending on what your paladin level was before.
None of this is consistent with items or abilities that grant you "+1 caster level", which is something entirely different.
Levels in a spellcasting class are not class level, and they are not caster level...they are what level you are considered to be for determining spells per day, spells known and caster level in one particular class. In the case of PrCs, they are the class level of your base spellcasting class PLUS whatever advancements you get from the PrC...NOT PrC class levels, only those which add to your spellcasting class.
A class level in a PrC that does not advance your spellcasting in a prior class is NOT a "level in a spellcasting class"; in fact technically, NO PrC class levels are "levels in a spellcasting class" (for the reason Frank pointed out, with Mystic Theurge shenanigans). What PrCs DO do is increase your "levels in a spellcasting class" for a different class.
If you are a Wizard5, you have character level 5, class level 5, caster level 5, and 5 levels in a spellcasting class.
If you are a Wizard5/Fighter5, you have character level 10, class levels of 5 and 5, caster level 5, and 5 levels in a spellcasting class. If you take Practiced Spellcaster, your caster level goes up to 9...but nothing else changes.
If you are a Wizard5/Fighter5/Eldritch Knight5, you have character level 15, class levels of 5/5/5, caster level 9, and 9 levels in a spellcasting class. If you take Practiced Spellcaster, your caster level goes up to 13...but nothing else changes.
You cannot take a PrC to advance your Eldritch Knight casting, because you don't HAVE Eldritch Knight casting...it is not a spellcasting class.
*****
Now: to the specifics of Ur-Priest. It says:
To determine the caster level of an Ur-Priest, add the character's Ur-Priest levels to one-half of his levels in other spellcasting classes.
Breaking this down...
"...the caster level of an Ur-Priest..." This is referring to the level at which his spell effects are calculated; it has nothing to do with his class level (BAB, Saves, etc) OR his levels in a spellcasting class (spells per day).
"...add the character's Ur-Priest levels..." This is referring to his CLASS level in Ur-Priest (this format is consistent in the books; when it says "(Derpmaster) levels", it means class levels. This clause does not refer to caster level or levels in a spellcasting class.
"...to one-half of his levels in other spellcasting classes." This is specifically referring to the level that determines spells known/spells per day, specifically excluding Ur-Priest (use of "other"). This means that any level in any other class the character has that is added together to determine spells per day counts.
Let's take an Ur-Priest 1 and look at his (possible) past levels:
Wizard5/Fighter5/Eldritch Knight5? 9 levels in a spellcasting class (because you cast as Wizard 9). If this guy had Practiced Spellcaster, his caster level (for Wizard spells) would be 13, but he would STILL only have 9 levels in a spellcasting class.
So his class level (in Ur-Priest) is 1. He gets spells per day as an Ur-Priest1. His caster level for Ur-Priest spells is 5 (half of 9, +1). His character level is 16.
Paladin5/Monk5/Sacred Fist5? 9 levels in a spellcasting class (because you cast as a Paladin9). Even though your caster level (for Paladin spells) is only 4.
So his class level (in Ur-Priest) is 1. He gets spells per day as an Ur-Priest1. His caster level for Ur-Priest spells is 5 (half of 9, +1). His character level is 16.
Fighter5/Pious Templar5/Shining Blade of Heironeous5? 7 levels in a spellcasting class (because you cast as a PTemplar7. It doesn't say how to figure CL for a Pious Templar, so I assume it's class level...so his caster level for Templar is 7.
So his class level (in Ur-Priest) is 1. He gets spells per day as an Ur-Priest1. His caster level for Ur-Priest spells is 4 (half of 7, +1). His character level is 16.
Wizard5/Druid5/MysticTheurge5? 20 levels in a spellcasting class (because you cast as a Wizard10 and Druid10), regardless of whatever the fuck your caster level is in either of those.
So his class level (in Ur-Priest) is 1. He gets spells per day as an Ur-Priest1. His caster level for Ur-Priest spells is 11 (half of 20, +1). His character level is 16. If he took Practiced Spellcaster (Ur-Priest) he could have CL 15.
If this looks like Mystic Theurge (using non-cleric) is the best way to stack up your levels in spellcasting classes for Ur-Priest, you would be right...since it gives you effective levels in 2 spellcasting classes at a time. I honestly do not know what to do if you Mystic Theurge with cleric...in the example above, you'd have Wiz8 and Clr8 casting, so that's 16 levels in spellcasting classes...but it says not to count Cleric levels of ex-clerics. But does that refer to Cleric spellcasting levels (8) or Cleric class levels (4)? I could see a good argument either way.
You could do Ultimate Magus for similar (if less potent) shenanigans.
Please give me an example of how this interpretation does not hold water, or is inconsistent.
Recap of problems with the "levels in spellcasting classes" = "caster levels" stance:
1.) PrCs clearly say "add a level in a spellcasting class", and give you things that are different from something that says "add 1 caster level".
2.) If you are a paladin6 and take a level in a PrC that gives you "+1 level of a spellcasting class", your casting ability goes up to paladin7...despite the fact that this does not change your paladin caster level at all (it remains 3). Unless these advocates are saying that the PrC should jump his casting to paladin8 (CL 4).
3.)
Frank wrote:Getting a 6th level spell slot isn't a feature of your level in spellcasting (which is your caster level), it's a class feature of your class level in Wizard.
No it isn't, as every goddamn PrC ever published proves. They don't advance your class level in wizard...they advance your level in wizard for the purposes of casting spells only...i.e. your "levels in a spellcasting class", just like it says in the goddamn chart.
I am judging the philosophies and decisions you have presented in this thread. The ones I have seen look bad, and also appear to be the fruit of a poisonous tree that has produced only madness and will continue to produce only madness.
--AngelFromAnotherPin
believe in one hand and shit in the other and see which ones fills up quicker. it will be the one you are full of, shit.
--Shadzar