GetIsClassDefault

The precise behavior of this condition function is unknown.

Reverse-engineering suggests that it takes a Class as its parameter, and checks if an unidentified field in the player-character's data is a pointer to that Class; it returns 1 if so, or 0 otherwise. (If the parameter is not a Class, then the function behaves as though the parameter were None.) This unidentified field is written into the save file, but it's unclear what, if anything, ever actually modifies the field, and the field defaults to None.