Without taking a meter to it and knowing this has its own speakers, it's likely an extension speaker jack as the built in isn't very powerful.
The built in speakers are actually surprisingly powerful. It can get pretty bass heavy on it's own especially when the actual bass floor pedals are working

It has a 12" 10" and 6" speaker which is an interesting set up, not sure if they are all always on though as one is an acoustic trem speaker or something. The 9500 has an extra speaker too, the 9000 and 9100/9200 only have 2 speakers and I believe the 9500 has the extra 6" which may not be much of a difference but it helps.
The output is a headphone jack for sure, I wish it had an extension jack too. I guess the power out and leslie out are extensions though.