The time that you enter into Zola Suite is called actual time. If you are billing using an increment, billable time will be the amount of time you bill your client once it has been rounded up using your increment.
For instance, you work on drafting a document that takes 25 mins and your rate for the matter is $100/hour with a 6 min increment. Your actual hours will be 25 min but your billable time will be 30 mins because the actual time is 25 min which is 1 min more than 24 (a number that is evenly divisible by 6). Thus, Zola Suite rounds up your actual time to the next number that is divisible by 6, to make your billable time 30 mins.