Backblaze which number is which might help, although manual’s B2 Cloud Storage could use an update… This is a somewhat new added feature of B2, so various things are still catching up with change, I suspect.
Application Keys is Backblaze’s attempt to explain it. Basically, there are now two flavors for an ID+key pair. Using either kind should work, but the new application key (and associated keyId) is better security practice.
Locating the Master Application Key and Master Application Key ID Pair may also help with how to get them.