The Passcode B is a key item in Resident Evil 6 in Ada's and Chris' campaign.
This passcode is needed along with Passcode A and Passcode C to go through closed doors in the Forward Hangar.
In Ada's campaign, this passcode can be dropped by one of the J'avo guards in one of the rooms, but the passcode would be taken by the Rasklapanje. Ada must chase and kill the monster in order to get this passcode.
It can be found In Chris' campaign by Chris when they are separated with Piers when Rasklapanje attacks them.