When I was a very young officer, back when the earth was still cooling:
There were many names for "dinners" -
- "Dining in" generally referred to an informal event involving the officers (or NCOs) of one unit only - it was a multi-course meal and each unit had its own rules (including when and how a member might leave the table before the loyal toasts), dress was, generally, patrol dress for serving officers and black tie, (dinner jacket, tuxedo) for retired officers;
- "Mess dinners," sometimes called "guest nights," were a step up - more courses, more rules. Dress was mess kit for serving officers and white tie (tail coat) for retired members and civilian guests.
- Ladies nights were of various levels of formality - I recall attending a couple low very formal ones - but the rules were relaxed.
Uniforms were worn only by serving members - retired members were expected to own both black tie and white tie, both fitted for medals.
Civilian, formal, highland dress was very popular because it was usually a bit cheaper than black tie plus white tie.