An optional action may be performed when new composite members first touch existing members. This action is stored in the $OnJoin action. When a note that was not previously a member of a composite is dragged to touch a note with an $OnJoin action, the $OnJoin action is performed on the dragged note. If a note touches more than one note in the composite, each $OnJoin action is performed in turn. When an $OnJoin action is run, the 'this' designator is bound (refers to) to the note newly joining the composite and 'that' is bound to the existing composite note running the $OnJoin action.
See also—notes linking to here: