Stupid question re: speaker out jack

Not in front of my HD212 at the moment, but there is a jack on the back that is marked "external speaker" or something. What ohm rating is this? Would this allow me to use another 212 speaker cabinet? Would I then switch the OHM rating from 8 to 4? Thanks in advance for any help.

Your jack should be a series jack. It SHOULD say that on the back, by the jack somewhere. If it does, normally you will use the 4 ohm side for internal speaker only and 8 ohm for when you add a speaker. Mike.

There is no note of it being a "series" jack. I'm assuming that the plug would go to an 8 Ohm cabinet and then I'd switch to 8 Ohm but then I'm also getting confused about the wiring on the built in speakers:
Does the wiring have to be in parallel on the built in 212 speakers? Another stupid question -- how would I tell? Why would they even have the jack there if the wiring to the speakers wouldn't be set correctly? Thanks in advance!