From the mid 1890's and possibly earlier, older carriages were stored and used infrequently. Their use was generally to provide more capacity on trains at holiday times or for the make up of special excursion trains.
The addition of a letter 'H' next to the code, but not part of it, was the initial indicator with the 'H' later becoming part of the code from the 1900's. By 'indicator' I mean the code was B with 'H' an indication of traffic. This is determined by correspondence and Register entries which tend not to include the H in all reportings.
Carriages marked BH were downgraded from class _B__. For some carriages it is difficult to determine whether they were converted from carriages as B and came BH or whether the conversion was directly to BH.
The BH cars were recoded and renumbered to _YH_ in the 1910 recoding.