First of all, in the past I think it has depended on the partner. So I don't know if it could be answered in general like that.

Second, isn't the prefix being phased out? (As a US-based member Mine used to start with GP- (not GPX-) for ages (because I opened the account way back when it was called Guest Privileges!). Then lately they stopped displaying the prefix part altogether on the Choice webiste. A couple years ago when I was registering for promos, this was even confusiong within Chioce!

(My latest physical membership card is from the start of the year, and it still has GP- on it. But when I log in, and it takes me to My Account page, the Membership Number it displays there is the same one I've always had except without the GP- on the front. (I seem to recall this only started fairly recently, though. And I can't say whether it's true for everyone, I just know it's true for me.)

The even bigger question is whether every partner knows all the formats of the Choice member number that are used worldwide. I've certainly had analogous problems before with, for example, Wyndham Rewards not recognizing some Frontier Airline account numbers after Frontier merged with Midwest Airlines (who had a different account number format). And then there's the endless confusion since the dawn of time as to when to include and when not to include lading zeros (of which there are so many!) in Southwest Airlines account numbers.