Return Code	Significance
0	action was registered
1	action was not registered

The function passed will be called after a selection-tool button at the field has been clicked on.

If the function is called several times with different function pointers, the functions passed in this way form a stack, which is executed at the trigger moment. If one of the functions in the stack delivers a return code other than zero, the next trigger function in the stack will be jumped to (no mouse action on selection tool button registered). If the return code = 0, the execution of the function will be terminated.