There is a List Callback resource for each of the selection types (e.g. XmNsingleSelectionCallback and also a XmNdefaultActionCallback.
The selection policy Callback will be called first and the the default.
A list item may action a callback by double clicking on his selection.
The Callback function has the usual form:
list_cbk(Widget w, XtPointer data, XmListCallbackStruct *cbk)
Element of the XmListCallbackStruct that may be of use include:
See reference manuals or <Xm/List.h> for complete details.