I take it we talking about an amp and speaker in a box, with the internal speaker permanently connected, and a separate impedance-selectable output for an external speaker.

Quite simply, the impedance plugged into this output should match (or be more than) the value marked on the socket, so you can plug a single 16-ohm speaker into the 16-ohm socket, or two paralleled 16-ohm speakers into the 8-ohm socket. Or two 4-ohm speakers in series into the 8-ohm socket, come to that. The impedance of the internal speaker shouldn't come into it.