When Time off in Lieu (TOIL) is requested in Appogee, the amount of time specified is added to the user's allowance. TOIL doesn't deduct from the user's allowance, it reimburses the allowance instead.
Once a TOIL request has been approved, the user can then request leave (as normal) and the amount will be deducted from their allowance.
For more information on what is TOIL and how to set it up, please see here.
Please note: It is possible to rename TOIL for your organisation as some companies may refer to it as Comp Time/Compensatory Time Off. This can be configured by an administrator via System Config > General Settings > Leave Settings tab.