I understand the need to have referential integrity for limiting specific values on entry or possibly preventing them from removal upon a request of deletion. However, I am unclear as to a valid use case which would exclude this mechanism from always being used.

I guess this would fall into several sub-questions:

1. When is referential integrity not appropriate?
2. Is it appropriate to have fields containing multiple and/or possibly incomplete subsets of a foreign key's list?
3. Typically, should this be a schema structure design decision or an interface design decision? (Or possibly neither or both)

Thoughts?