Deductions in
Rake Tracking general mean bonuses that poker rooms pays you. These bonuses are deducted from your gross rake to get to net rake out of which
rakeback is calculated. You see, rakeback is paid our of the
profit that you generate to poker rooms, so any free money in form of bonuses usually gets deducted from your rake.
In case of
Full Tilt rakeback they make a number of other deductions as well, like deposit fees. You can view a list of these deductions from the rake tracking Full Tilt detailed view.