This is the underside of Border's turret. They have put two slots (arrowed) in the bottom edge of the wall. These slots may be invisible when the kit is assembled, in which case you may ignore this note; or they may "punch through" the plastic and be visible from outside. In that case, you could fill them to improve accuracy.
The real "Initial" Tigers had two round holes in those positions. These holes were invisible from outside. It was only later on that slots were made here, to simplify production.