The code is usually printed near the top of your notice, next to the description of the contravention. Below are the most common national codes in plain English, with the appeal angle for each, the bit councils don’t tell you.
Your notice is definitive. These are the common national codes; the exact code and wording on your own notice always take precedence, and some authorities use extra codes or suffixes. The full official lists are published by PATROL (outside London) and by London Councils, and on your council’s own website.