You can request and be granted annual leave which is expected to accrue by the end of the leave year.
You must request advanced annual leave via the webTA Leave Module or by submitting an SF-71 annotated to show "advanced annual leave." Your supervisor may or may not approve the advance; they need to have a reasonable assurance that you will be in a pay status long enough to repay the advance. If it is known that you will not be in a duty status long enough to repay the advance, then it can’t be granted.
If you have been granted the maximum amount of advanced leave, then you can’t be granted any more. If you are subsequently required to be absent from duty and other types of leave are not appropriate, then you may request leave without pay for the duration of the absence.