Types of GMS Callback Procedures

Activation callback procedure: A G2 procedure that is called when the user selects a GMS menu leaf entry. The activation callback procedure associated with a particular menu entry is specified by the Gms-activation-callback attribute of the entry's template object, as described under Specifying the Effect of Choosing a Leaf Entry.
Posting callback procedure: Allows you to dynamically configure any statically defined menu bar or transient menu. GMS invokes this procedure immediately before it displays the menu, and again immediately after it hides the menu. The posting callback procedure associated with a particular menu is specified by the Gms-posting-callback attribute of the root template or cascading menu template as described under Invoking a Procedure When a Menu is Displayed or Hidden.
Selection callback procedure: Allows you to take an action when a statically defined menu entry is selected or unselected. A selection callback procedure is similar to a posting callback procedure, except that it is called on selection and unselection rather than on display and undisplay. The selection callback procedure associated with a particular menu entry is specified by the Gms-selection-callback attribute of the entry's template object, as described in Invoking a Procedure on Selection and Unselection.