Spell Circles

We use the terminology ‘spell circle’ instead of ‘spell level’ to disambiguate from character level. A fifteen level wizard is a eighth circle caster; Mind Blank is an eighth circle spell; to cast Haste you need to be fifth level, as it is a third circle spell.

Spell circles are diegetic; people in-universe (who can make an appropriate knowledge check) know that there are different spell circles, that spellcasters can only cast so many spells of a given circle in a day, and that a given spell is (almost) always the same circle for all casters of the same type (All wizards cast fireball at third; all clerics cast plane shift at fifth, etc.).

Discrete character levels are not diegetic. No in-character distinction is drawn between a 5th-level wizard and a 6th-level wizard. Characters may notice that the latter’s spells are slightly stronger, and guess accordingly that the latter is closer to achieving 4th circle, but things like “caster level” are continuous rather than discrete phenomena.