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Surviving records generally comprise forms, or cards summarising general correspondence. These contain some genealogical information (date of birth, details of spouse, next of kin) but mostly deal with postings, service abroad, injuries or wounds, and personal conduct.
A variety of forms or cards may be sent to you including enlistment documents (Army Form B284), which may include brief records of service, Regimental Conduct Sheets (recording disciplinary offences like drunkenness or overstaying leave), Medical History and Dental Treatment sheets.
Of greatest use in tracing service is the Service and Casualty Form (Army Form B103). This gives basic personal information, then details promotions (acting temporary, local or substantive), appointments, transfers, postings, attachments, forfeiture of pay, wounds, accidents, admission to and discharge from hospital, Casualty Clearing Stations and so on. Date of disembarkation and embarkation from a theatre of war (including furlough) are also included. The whole basic structure of the soldier’s career is here!
There may also be an Army Form B102, which contains similar information to the B103, but usually in more condensed form. There may also be Discharge Papers, which provide some details of where they served, along with a testimonial (brief written note by an officer saying how they conducted themselves).
Don’t expect to find anything about what the unit your relative served in was actually doing – for this sort of detail you’ll need to consult their War Diary. The service record should provide enough information to help you trace the relevant diary(ies).