Advance class booking determines how far out in the monthly calendar your clients can click to see what class sessions you have scheduled. This setting is only for your clients; you can look as far out as you'd like to when you are setting up appointments using TimeTap's Back Office.
To get to this setting, you'll want to go to Settings → Time → Class Time Display:
By default, the advanced class booking is set to 3 months, which means if today's date is July 17th, 2015, I would not be able to see any of the class sessions that are scheduled after October 17th, 2015:
If I change my advanced booking to 4 months:
Then I would be able to click through all the way to November:
Advanced class booking also determines how far our a repeating schedule is created for your class. If I have a class schedule set to and end date of "Forever" so that it shows as ongoing, then class sessions will be created for as far out as my advanced class booking allows.
In the example below, I have my TimeTap account's "Advanced Class Booking" set to 3 months and my repeating class schedule set for every Tuesday from 7/21/15 through Ongoing:
Thus, if today's date is 7/17/2015, Individual Class Sessions would have been created for that repeating class schedule starting 7/21/2015 through 10/17/2015. The last session date that could have been added was 10/13/2015 since that is the last Tuesday before the 3 month cut off date of 10/17/2015: