As stated the Special Connecting Cable is usually missing from the 11 or the 11/12 combos people buy. Nice that you have it.
The cable is needed to power headphones in the 11
The cable is used to be able to use the speaker selector switches on the 11 and
The speakers connect to the barrier strips below the connector, one column for each set of speakers
The use of the cable is optional as many don't need headphones and with one set of speakers connect them to the amp
The use of the cable allows switching speakers off when using cans.
You can use the cable only for the headphones and connect the speakers to the amp but that will mean the speakers play all the time, less convenient.
This feature with the cable is a throwback to the amps in the cabinet of old mono and early stereo systems. Many preamps from the early 70s have this, most preamps from the late 70s and on do not have it. Amplifiers frequently added speaker switching for some companies.
Nice combo, enjoy.