The API attempts to rigidly define its interface, including the possible data returned. However, in a few places the dynamic and changing nature of the Stack Exchange network doesn't allow us to fix certain returned values.
In API V1.0, we errored on the side of not returning new values. However, this prevented us from exposing new things in the network like the Stack Exchange Chat sites. Since V1.1 we have unsealed a few "enumeration style" fields explicitly, to allow developers to incorporate additions to the network without waiting for a full API revision.
All "enumeration style" fields have their possible values documented as part of their containing type's documentation. Unless note is made to the contrary, API consumers can safely assume that those listed values are all the API will return.