MIDI clock sends all the time, I don’t think it can be turned off. If set to auto, it will check for external clock first and sync to that, otherwise it uses internal tempo, set with [MANUAL] and then  or . That means any synth that receives output from the JX will need to sync to it or set up local control so that incoming midi doesn’t affect it.
Along with MIDI clock, the sequence always send its midi notes to the MIDI output. It will be important to never start a sequence from the JX because it will probably send unwanted notes onwards.
Chain mode was set to on at first, and this caused problems because it would not send note offs from the MIDI output, which caused hanging notes.
The JX also sends program change messages, which cannot be turned off. This should be noted for any downstream synths in the chain.
Some USB ports do not supply enough power to keep the JX on. My phone charge battery for example did not work.
Not sure about stereo, if the JX receives and/or sends it on with its input/output jacks. The output on the JX is monoaural, except for the chorus output, which is stereo.