"My" US Cards all start with "GPX-" (not with my initials, i.e. ZF). "My" Canadian cards all stat with "CN-".
The funny thing is, when I log in, it shows the numbers without prefix, i.e. without "GPX-" or "CN-".
When I go to "contact customer service - send e-mail", it automatically shows the numbers including the prefixes

That really confuses me and makes me think that the prefixes are part of the number.
The reason why I am asking it that I get for my Topguest check-ins with other programs (e.g. Best Western or HHonors) my points after two days on my accounts. Not so with Choice. I get the all the e-mails "you've got 50 points" but in fact, they don't show in my Choice accounts