Boss Raptor's almost hit the nail on the head. Hypothetically, just suppose Air Miles sell you a ticket for GBP 59 return to Paris with BA. On the day you're due to go, British Airways suffer massive problems and cancel your flight - revalidating your ticket for an Air France flight later that day.
The term "Uplift Value Restricted" indicates to Air France that they will receive a maximum of 50% of the GBP59 paid for the ticket (the one-way fare actually paid for the ticket)- even if the only seats left available for sale are at the Full Business Class fare (close to GBP250!)
Fortunately for the travellers of scheduled airlines, IATA "Gentlemen's agreements" exist where we all rally round and cooperate to help each other's customers in times of crisis - sometimes the welfare and convenience of our clients come before pure financial issues!