Leaking quarter turn tap. Cartridge removed, cleaned, lubed and now much better. Stamped in the cartridge was BS5412 and CD XZ 20.

Does the CD XZ 20 uniquely identify the size of the replacement, or is there more stuff I need to know before ordering?

No idea on the code, if you don’t know the tap manufacturer and or model then you’ll need to do some counting and measuring.
This link will identify it for you
https://shop.lunns.net/ceramic-disc-valves-42-c.asp

Having just been through all of this with a code that just turns out to be the month of manufacture, I sent a picture to tapmagician.co.uk that solved it in no time.