Diatonic functionality

In music diatonic functionality is the assumption used in tonal music theory that a pitch or pitch class and its enharmonic equivalents can not be used in the same way. For example a C# cannot be used as a Db, however this is not to say that they are not enharmonic equivalents, for they are in equal temperament, but rather that a Bb minor chord does not have a C# as its third, but a Db, and an F# major scale does not have a Db as its fifth degree, but a C#.