This structure contains UDF parents information:

This information consists of UDF parents, parent's direction data and
parent's mapping data.

Instantiation:

old_refs: can be obtained from the UDF feature definition.
new_refs: can be inputted by the user.
reverse_refs_dir: can be inputted by the user.
mapping_data: some of them must be inputted by the user (please see all the
comments about the data structure UF_MODL_udf_mapping_data_t).

Edit (from a UDF instantiated feature):

old_refs: can be obtained from the UDF instantiated feature.
new_refs: can be inputted by the user.
reverse_refs_dir: can be inputted by the user.
mapping_data: some of them must be inputted by the user (please see all the
comments about the data structure UF_MODL_udf_mapping_data_t).

Ask (from a UDF instantiated feature):

old_refs: can be obtained from the UDF instantiated feature.
new_refs: must be NULL.
reverse_refs_dir: can be obtained from the UDF instantiated feature.
mapping_data: can be obtained from the UDF instantiated feature.