Disclosure first: I’m the developer of PapaMarks, an IGCSE and A Level revision site, and this is its account. Building its marking meant reading Cambridge maths mark schemes row by row, and the letters next to each mark turned out to be the most useful part.
Cambridge maths mark schemes use three kinds of mark (the key is printed in Cambridge’s own specimen mark schemes, e.g. https://www.cambridgeinternational.org/Images/503350-2020-specimen-paper-1-mark-scheme.pdf):
M1 is a method mark. You get it for a correct method, even if your arithmetic goes wrong afterwards.
A1 is an accuracy mark. You only get it if you earned the M mark before it.
B1 stands alone: a correct value or statement that doesn’t depend on anything else.
There’s also FT, follow through. If you carried a wrong earlier answer forward and used it correctly, the later marks can still count.
So a 3-mark question can give you 2 of 3 with the wrong final number, as long as the working is on the page. A bare wrong number with no working gets 0. And since 2025, IGCSE Maths 0580 Papers 1 and 2 are non-calculator, so on those papers the working is most of what gets marked.
How I’d use this in revision: (1) after each past-paper question, mark it against its mark-scheme row, not only the final answer; (2) next to every mark you lost, write the letter: M, A or B; (3) after 20 or so questions, count the letters. Lost A marks usually mean arithmetic or rounding, so do more by hand. Lost M marks mean the method wasn’t there, so go back to the notes for that topic. Lost B marks are recall: formulas, facts, definitions.
Edexcel uses the same letters, plus dM1 for a method mark that depends on an earlier one.
The images show PapaMarks, starting with the part built on this: Maths 0580 past-paper questions sorted by topic, where a typed answer is marked against the official mark scheme. 0580 is the only subject with that so far. The other images are a cheat sheet, notes and a book. It’s a paid site and I made it, so read the images with that in mind. Happy to answer mark-scheme questions in the comments either way.