You’re asking about a changeset made 12 years ago and apparently I couldn’t recall the circumstances. I guess MTR did advertise B1-B4 exits at some point before they were replaced with a simple B. FYI B1-B4 are shown in the Buildings Department’s railway protection boundary maps: https://www.bd.gov.hk/doc/en/resources/codes-and-references/ScheduledAreas/MTR-RP-59.pdf