If more than one symbol is located immediately adjacent to another, it usually means that a multi gang box for multiple wiring devices and are to be installed.

It usually means that a multi gang box for multiple wiring devices and A SINGLE COVER are to be installed. This sign is necessary in order to ensure that the electrician during installation used the right quality and size of box to enclose the number of wires and devices required, taking into consideration the specifications about the number of wires and devices permitted in a common box.