It is currently patched to BPD CH-1 460.350 allowing agencies with VHF radios to hear radio traffic related to the events in the Boston Common area. When there are no special events you will hear BPD BOLO's and all channels announcements simulcast on the channel.
158.9100 began as channel for the Academy Video Unit in the 80/90's
It them migrated to being the Boston Federal (since most of the FEDS they were dealing with are/were VHF) Interop. Heavily used/implemented during the Boston DNC
Today you will find it regularly patched to BPD UHF, as well as (for events) a couple of Federal IO (Google NIFOG) Repeaters and is a regular on Boston All-Out Broadcasts when the Dispatcher Selects ALL Channels (except BAPERN)