noun
The letter B itself, used as a noun (e.g., grades, blood types, musical notes, or labels).
彼の成績はオールBだ。
His grades are all B's.
血液型はB型です。
My blood type is B.
noun
basement floor
Abbreviation used in floor numbering, as in B1F (basement first floor).
会場はB1Fです。
The venue is on the B1 floor.