The FAQ section of the manual has an entry on possible values of reserves/old_reserves Found values: https://koha-community.org/manual/24.11/en/html/faq.html#reserves-holds-table It is missing one - P, which is applicable if library has HoldsNeedProcessingSIP system preference enabled. The reserved item has been returned using a self-check machine and reserve needs to be confirmed by librarian before notice is sent and status is changed to waiting.
The Documentation team made the decision (some time last year?) to slowly but surely get rid of the FAQ page. Instead, we want the information that is still relevant to be moved to the corresponding chapters. When someone adds the P status as explained by Hebah, could you please move the section as well? At first glance, I'd suggest moving the whole of the FAQ > Reports > Codes stored in the database to the Reports chapter - but happy for a better location to be used.