It decides based on a few things, slightly different flow logic for each form.
ADD / UPDATE is decided based on the record being on the DB matching the table key or not, FI values or not etc
I think the default is update mode and the forms engine then determines if the key is there on the DB/not or of the FI have values or not etc
As for copy etc, any form interconnects placed in a copy button will open a called from in copy mode
The form type "decides" it, Rauf.
A Work With will always BROWSE the records, whereas Fix Inspect, Headeless Detail & Header / Detail will activate the ADD/UPDATE?COPY modes.