Average Manna Level world. Areas of higher or lower manna, or aspected manna and special effects may exist.
Arcane vs Clerical:
Arcane magic as practiced by mages works exactly as presented in GURPS Magic.
Clerical magic as from the Power Investiture advantage is a bit more narrow in scope, but able to reach certain higher “level” spells (prayers) more quickly in many cases.
A cleric’s patron diety or power usualy has a few magic colleges related to their nature that they specialize in, and often will allow the learning of other spells/prayers that align with their purposes.
Prerequsite spells from outside the granted colleges are not required, much like “one college magery”.
The trade off for this is that clerical magic is less versatile than arcane magic, and the cleric must select spells appropriate to their patron as judged by the GM. Also they must behaive in a manner congruent with their patron’s desires or risk loosing their granted abilities.
Clerical magic relies on Sanctity not mana and is unaffected by mana levels except for the fact that it will not work in a No Mana zone unless possibly if that same area is a High Sanctity zone for the cleric’s patron.
Special Note: Healing
Almost all clerics have access to some Healing college magic, but clerics whose patron includes the Healing college in their portfolio do not suffer the penalties for multiple castings on the same individual.