Check-in baggage refers to the baggage provided by the airline for that particular itinerary. It does not cost anything and comes within the net price of the flight booking.
Baggage selected under SSR is the extra baggage you select that is chargeable. It is an add on to the check in baggage provided by the airline
Check In Baggage – 15 KG
SSR baggage opted – 10 KG
Total baggage permitted = 25 KG