What did the letters mean on side of a Spitfire?

The letters on the side of a Spitfire typically represent the squadron or unit to which the aircraft belongs. These letters are usually followed by a number, which indicates the specific aircraft within the squadron. For example, a Spitfire with the letters "PR-S" on the side would belong to the Photographic Reconnaissance Squadron, while a Spitfire with the letters "609 Sqn" on the side would belong to the 609th Squadron.