The default behavior is not to allow selection of owned objects. If you pick
an owned object, the UDO object is actually selected. If the owning UDO
object is not eligible to be selected, then the owned object can not be
picked.

The UDO can be changed to allow the selection of an owned object. In this
case picking of the owned object will select the owned object and activate
the up one level button, if the owning UDO is eligible for selection. Now
the owning UDO can be selected using this button. If the UDO is not eligible
for selection, picking of the owned object will select the owned object only.

The selection behavior of the UDO class can be changed by using the routine:
UF_UDOBJ_set_owned_object_selection

UDO's can own other UDO's. So you can have the chain:

UDO1 owns UDO2 owns UDO3 owns point1

For a detailed discussion, please see Selection of UDO's that own other UDO's


Interactively selecting owned objects for deletion is specifically disabled.
So even if the UDO class is set for UF_UDOBJ_ALLOW_SELECTION, owned objects
can not be interactively selected for delete through Edit->delete.

If the owned object is a solid, then the solid may still be selected through
modeling selection where a feature list is presented to the user. So for
example, it may be deleted through Edit->Feature->Delete Feature. This is
because ultimately modeling is in control of all solid bodies.