BPSO is Base Personnel Staff Office (ie the department responsible for personal records, pay etc of the airmen and officers of the group it was supporting - Group is the level of command above Squadron) and the HS means held strength. This means that for some reason ('C') he was attached to this office for administrative purposes while on unspecified duties or leave of absence. It is possible that the C stands for Compassionate, ie some sort of personal or domestic problem. However 8 months (from 18/3/44 to 25/11/44) is a very long time to be on compassionate leave, so C may refer to something completely different.
And the other abbreviation is not GSO, but GSU, meaning Group Support Unit (their role is explained
here). 84 GSU was part of Fighter Command and was based at
RAF Aston Down in Gloucestershire at that time. 84 Sqdn was a Bomber Command Squadron.