This is the Open C API interface to User-Defined Operations.

The user should use either an attribute (see uf_attr.h) or a User Defined
Object (see uf_udobj.h) to associate data with a User Defined Operation. If
the data is of type integer, double or string one can use either an attribute
or a User Defined Object. If the data is a link to another object and that
link must be considered for update and delete processing then a User Defined
Object is most appropriate. Attributes are simpler to use than User Defined
Objects but don't allow for automatic update and delete processing. User
Defined Objects do participate in update and delete processing.

See uf_attr.h and uf_udobj.h and the Open API User Documentation for more
information.