Your organization may want to allow employees to exceed their time off balance when needed, and later adjust it with the future accruals. By default, Web TimeSheet allows users to book Time Off even if they don't have sufficient balance available. This way, the time off balance will become negative once the booking is done.

However, your administrator can always set a Maximum Overdraw for your Time Off. This value determines how many hours / days of extra time off one can overdraw from the allowed balance.
If you set the Maximum Overdraw to '0', then the users will not be allowed to exceed their allowed time off balance than the allotment made.
If Maximum Overdraw for time off set to '2', then the users can exceed their balance by 2 days or hours depending on the time off set up.