Four regions, one developer account — but separate seller registrations
Walmart's US marketplace has the highest seller count and the fiercest competition of the four regions. You take the keys the connection needs from Seller Center's developer portal. The point to note: the regions are separate seller registrations; the keys you obtain for the US will not connect to the Canadian or Mexican marketplace. A separate connection is set up for each region.
The partner identifier field looks optional but is required on some endpoints. If you have it, filling it in is the safest choice; it appears in your Seller Center profile. Leaving it empty and having some calls work while others fail is the most confusing state a setup can be in.