"Assigned times and surfaces" is one of the three ways to schedule your brackets and/or round-robins in a tournament.
The "Assigned Times and Surfaces" of the matches is when you schedule your tournament from start to finish. Matches being scheduled to a time and a surface.
You can slide rounds/matches onto specific surfaces at specific times.
Players can receive notifications on their phone to tell them their opponent has been assigned along with time and surface of the match.
How to use the surfaces scheduler to "assign times and surfaces"