Minimum Level: You may take a prestige class only at character level 7 or higher.
One Prestige Only: You may have at most one prestige class.
GM Approval: Prestige selection requires GM approval.
Each prestige class lists what you must have to qualify. These can include:
Class levels (e.g., “Fighter 5+ and/or Ranger 2+,” or “Class X 6+ or Class Y 6+, and Class Z 3+”),
Skill proficiencies (minimum tiers),
Race/subrace or other tags/feats.
When a prestige level says it counts as [Class], treat that level exactly as if you had taken a level in that class for all of the following:
Hit Points: Gain HP using that class’s Hit Die and Constitution as normal.
Spellcasting: Advance spell preparation/known and slot/considered level as that class (respecting your slot-table choice rules).
Martial Mastery: Include the level as a level in that martial class for your Martial Mastery known/points formula.
Core Proficiencies: Advance Dodge, Armor, Initiative, Reflex/Fortitude/Will, and weapon/armor group proficiencies exactly as that class would at that level.
Multiclass math: Wherever your rules look at “your level in a class” (equalisation, considered level, etc.), use the counted class level.
In short: mechanically, a counted prestige level is a level in that class.
When you take your first level in a prestige class, that level is a dead level:
You do gain everything listed under the Counts-As rule (HP, spellcasting, Martial Mastery, all core proficiencies, weapon/armor profs, etc.).
You do not gain any features at that level:
No base class features from the counted class at that level, and
No prestige features at that level.
At prestige levels after the dead level:
You continue to apply the Counts-As rule, and
You gain the prestige class’s own features at the prestige levels where they are listed.
Some prestige classes lock which class they count as; others let you choose (e.g., “counts as Fighter or Ranger”).
If a choice is offered, you choose when you gain each prestige level (or as specified by that prestige).
Once chosen (if the prestige says the choice is permanent), follow that class for all mechanics that level.
Slot Table Selection: If you have more than one prepared-caster class, you choose one class’s slot progression once; that choice is permanent.
Preparation: You prepare/know spells for each class using your effective level in that class (including prestige levels that count as that class).
Considered Level / Slot Cap: Include prestige levels that count as a prepared-caster when computing considered level for slot caps, per your normal formula.