On_Select userexit in Select menus/button-areas or On_Event userexit in element selection state.
If you apply the userexit to relation masks, DataView will only copy the relation record into the mask. DataView does not create a new record in the master tables of the depending entities.
Attention! Blob field contents are not copied.
Example:
In DataView itself the userexit is used for the standard function copy in the Select menu.