Defines the surfaces of a feature. These are used by the ask, create, and
set routines for tolerance feature. They are also used to define the
origin surfaces for directed dimensions.

Functions affected by the modification of this structure are:

UF_GDT_ask_directed_dimension
UF_GDT_ask_feature_parms
UF_GDT_create_datum_identifier
UF_GDT_create_fcf
UF_GDT_create_instance
UF_GDT_create_size_tolerance
UF_GDT_set_directed_dimension