Reminder: Maximum Credit Window can be placed above or below Prefer Off and Avoid bid preferences in a Pairing bid group; however, it must be above all Award bid preferences. This bid cannot be forgotten and it will automatically be placed above your first Award bid preference, if you attempt to add it below an Award bid. The interface will not allow the bid to be moved below an Award bid, nor can an Award bid be moved above it.
It tells the N-PBS Scheduler to award you a block closest to the maximum allowed credit as possible