For some reason, RN uniforms are/ were numbered. Those that I was aware of (and can still remember!) were:
No 5: one’s ‘best’ uniform, worn on special occasions. Officially known as ‘undress’, this used to be the normal working dress uniform.
No 5a: I only ever wore Battle Dress at Dartmouth. Black serge trousers with a (strangely, attached) serge blouson jacket, white shirt, black tie.
No 8: ‘working dress’ worn at sea; Navy blue cotton trousers, blue shirt with breast pockets, with or without a ‘Wooly Pully’ (sweater).
No 12: Black twill trousers and white shirt (again, with or without a wooly pully. Black tie. No 12 is the normal working rig.
No 2b: High front black twill trousers, black waiscoat, white ‘golfball’ shirt, black bow tie, short black jacket. Commonly known as Mess Undress
No 2a: like Mess Undress, but with a stiff-fronted shirt and wing collar, white bow, tie white waistcoat and white mess jacket. This is Mess Dress
No 2c: Red Sea Rig is the informal evening mess dress worn on board ship in the evenings: white short-sleeved (tropical)shirt with shoulder boards, black trousers, and a black or customized cummerbund
Funnily enough, the terminology used didn’t always use numbers. I remember Battle Dress, No 5’s, No 8’s, No 12’s, Red Sea Rig, Mess Undress, and Mess Dress.