Target "Track: Solo/unsolo"

Soloes the track if the incoming absolute control value is greater than 0%, otherwise unsoloes the track.

Behavior menu

See the REAPER user guide for details.

Solo in place: Soloes the track while respecting REAPER’s routing. This is REAPER’s default and since ReaLearn v2.4.0 also ReaLearn’s default.
Solo (ignore routing): Soloes the track muting everything else, no matter the routing.
Use REAPER preference: Follows whatever is set in the REAPER preferences.

Learning this target by pressing the "Solo" button of the master track is currently not possible but of course you can just select it manually in the dropdown menu.