Many attributes (including the UseDefs attribute itself) are unaffected by the setting of the UseDefs attribute, and default values will always be used without error for such attributes. The "Applicability:" section below lists the attributes which are affected by the setting of UseDefs.
Note, UseDefs only affects access to attributes internally within AST. The public accessor functions such as AST_GETC is unaffected by the UseDefs attribute - default values will always be returned if no value has been set. Application code should use the AST_TEST function if required to determine if a value has been set for an attribute.
AST A Library for Handling World Coordinate Systems in Astronomy