Some functions always return the same value when called with the same set of arguments. These functions are called deterministic. This is important if you want to create a clustered index on a view or any index on a computed column, because you can create these indexes only if they use deterministic functions.
Most of the built-in functions are deterministic, such as
Some built-in functions are deterministic or nondeterministic, depending on the way you use them:
CAST is deterministic for every type ...