The /B seems to be used for limited releases featuring a bonus dvd, yes. As for the cat. numbers, it tends to be that the limited release is one number ahead of the regular release, plus a "/B", like the example below.
AVCD-31433/B Ninin Sankyaku / misono
AVCD-31434 Ninin Sankyaku / misono
The /B seems to only be used in the case of a limited edition though, as for example the three 'FFX-2 Vocal Collection' albums, which feature one cd + one dvd, don't have the /B.
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