A Domain is the concept of a certain part of the universe created and controlled by a certain deity or group of deities. All Domains have their own fauna of both angelic and demonic creatures, all of which work differently and may have different roles between each Domain. Most supernatural beings in a Domain cannot harm or directly affect any sort of being from another Domain. Extremely powerful beings such as gods and arch-demons are however not bound by this restriction, and can interact with other beings as they please.
Each Domain works differently depending on how its deity or deities have decided it to be, which means that although they all look and feel just about the same, the way in which the supernatural works may be widely different. In some it may be a common thing for gods, angels and demons to be spotted and seen openly, while in some they may not appear to exist at all. Many gods choose to give life to their Domains, most often in their own images, and with their powers protect this life from coming to harm - mainly at the hands of demonic beings. How they do this also varies; some act as unseen guardians who protect their creations without them ever suspecting a thing, while others create angelic champions to keep the demons at bay through battle.