Partial Wusool discount usually happens on rides taken at the end of a month when your discount balance might be running low.
This means that for your ride fare, you may only get the discount available in your balance instead of the full 80%.
Remaining Wusool balance: SAR 20
Ride fare: SAR 50
Discount on ride: SAR 20 (instead of the 80% = SAR 40)