A session-template is the beginning of every wikiloops jamtrack: Session templates may contain only one instrument, whereas a remix is a mix of a given jamtrack adding one additional instrument.
Please note, that again there should be only one additional instrument per remix!
You may play more than one take of your instrument (lets say two guitars) in one remix, but do not mix two different instruments into a remix.
Why may I only add one kind of instrument per remix? And when to use "Sequencer" or "Mix"?
When visiting any a wikiloops jam, the interface displays the participants and the instrument played by anyone involved.
There is no option for someone having played two kinds instruments at the same time - if you added two layers of p.e. Guitar, it is absolutely OK to upload that labeled as "Guitar" - if you added Guitar and Bass, there is two possible ways to go about:
Either, you upload a mix containing the guitar first, and post a second remix also containing the bass as an attachment to the first one - this way you will appear twice in the remix tree, and both remixes can be properly declared.
The second, less ideal option is to upload the track containing both guitar and bass right away. Since you will not be able to choose both instruments for one track, you will have to label the track as "Sequencer", which is the wildcard option to indicate that a track does contain various instruments.
As you may imagine, those tracks have to be excluded from any search results by lineup, because the system cannot tell which instruments are involved.
Another special case is covered by the ":Mixer"-declaration: If you actually did not add any instruments or sounds, but created a different mix of the tracks supplied by others, then you should choose this label for your upload.