The Genlock button on the CDTV remote is for mode selection of an
optional video genlock device that can be installed in the CDTV.  This
button has no equivalent on the A570.  If you want to support this
feature in an application running on an A570, you cannot be sure that
the genlock used with the Amiga supports software control of mode
switching.  Again, mention this fact in the user manual.