When you create or edit a Smart Tempo multitrack set, Logic Pro does the following:
Creates a downmix of the audio files in the multitrack set.
Analyzes the downmix using Smart Tempo analysis.
Writes the results of the Smart Tempo analysis to all the audio files in the multitrack set.
In addition, you can use Smart Tempo to create a group including all the tracks in the multitrack set, so they can be edited together.
The downmix is created from the multitrack audio files for which the Contributes to Analysis checkbox is selected in the Smart Tempo Multitrack window. The Smart Tempo analysis results for the downmix are applied to all files in the Smart Tempo multitrack set. The Smart Tempo analysis for the downmix can be viewed and edited in the Smart Tempo Editor, and the edits applied to all files in the multitrack set. For information, see PDF
Logic Pro Instruments: PDF