An index for the structural assembly being modeled.
This is an indicator to whether the whole assembly is modeled
or if only a subset of the structural assembly is modeled.
This data item is a pointer to _ihm_struct_assembly.assembly_id in the
IHM_STRUCT_ASSEMBLY category. The IHM_STRUCT_ASSEMBLY category provides the
details regarding the different structural assemblies used in the modeling.
The default value for this data item is "1", indicating that the entire
assembly is being modeled.