Domains

All spells fall into a certain domain, each ruled over by a saint of power. every domain has rituals, spells, and some level of requirements for using them. many spells will fall under different domains such as 'charm person' being used by both the saint of trickery and the saint of peace.
Understanding, the arcana domain has access to the most direct form of power. Nerru - Saint of Arcana is the keeper of secrets and the creatures of this domain are puzzle masters and collectors of knowledge. (they use Arcana and Knowledge Domains for 5e play)
 Forge, Grave, Life, Light, Nature, Knowledge, Trickery, Twilight, War, Travel,

Localization

certain spells will work better in places where that domain is stronger, a spell of the war domain would work better in a place that has seen violence like a battle field or a crime scene and life domain spells would work better in a hospital or a place known for healing properties already.