I’m often asked questions like this, “What can I use the extension speaker jack for?” or “can I use another cabinet at the same time as my internal speaker?”. The answer is yes for the most part and you’ll find that a majority of the classic Fender amps will have an extension speaker jack with the exceptions being the smaller amps like the Champ or Princeton.
Let’s use the 5E3 Deluxe as an example of what’s called a 100% mismatch. The 5E3 originally came with an 8 ohm output transformer (OT) as well as an 8 ohm internal speaker, but it also had an extension speaker output wired in parallel for adding another 8 ohm speaker cab to the amp. With the extension cab the amp would now have a total speaker load of 4 ohms (8 ohms + 8 ohms parallel = 4 ohm total load). That would be a 100% mismatch and totally acceptable with most Fender circuits. In fact, Fender has offered the extension speaker outputs on most of their amps for well over 50 years like that now.
While that may be an acceptable practice with most Fender circuits it’s not recommended for other amps, and you’d need to investigate that further for your particular circuit. In many cases you’ll find an impedance selector switch or separate outputs of different impedances to accommodate the use of various speaker loads. A good rule to go by would be to always make sure of the correct speaker load for the amp before plugging in cabinets.