Solo tracks in Logic Pro for Mac
You can solo a track, silencing all tracks that are not also soloed. Soloing tracks is useful when you want to work on a track individually, or work on a few tracks, without hearing the other tracks in the project.
The first time any track in a project is soloed, a large Clear/Recall Solo button 
appears in the area above the track headers. You can quickly unsolo or resolo all soloed tracks using this button.
You can also solo tracks (or channel strips) containing selected regions or cells. If no track is currently soloed when you invoke the Toggle Solo Tracks (Channel Strips) of Selected Regions/Cells key command, Track Solo (technically, Channel Strip Solo) is engaged on all tracks containing selected regions or cells. If some tracks are soloed when the key command is invoked, there are two possible results:
When all soloed tracks contain selected regions/cells (in other words, there are no selected regions/cells on tracks not soloed), all track solos are disengaged.
When there are either selected regions or cells on tracks not soloed, and/or there are no selected regions or cells on soloed tracks, solo is engaged on tracks containing selected regions or cells, and is disengaged on tracks containing no selected regions or cells.
The result is that the track solo state can be toggled on or off for a given set of selected regions or cells. In case the set of selected regions or cells changes, track solo is updated accordingly, before all track solos would be toggled off again. This works somewhat differently than region solo or solo lock, but combines the best properties of both operations in a single command, as the solo state is independent of the region or cell selection state after it is engaged.