If you create a section with a different time signature, JJazzLab will create the corresponding song part with an adapted rhythm. This is simply the rhythm from the previous song part adapted to a new time signature.
You’ll notice that the adapted rhythm does not need additional channels in the mix console: it uses the Midi channels of its source rhythm (fastbossa.s629.prs in the example above).
If you don’t want the adapted rhythm, juste replace it using the rhythm selection dialog.
You can't remove a source rhythm if its adapted rhythm is still present in the song. See example below.
Example: any operation (change rhythm, remove song part, change time signature of the parent section) which makes the source rhythm fastbossa.s629.prs disappear from the song will generate an error.
In the above case, if you want to replace fastbossa.s629.prs by another rhythm, you need first to remove the second song part which uses the adapted rhythm.