I have been reading through the free Security groups part of the documentation and the paid plugin’s too (https://store.suitecrm.com/addons/securitysuite) and I do not quite understand yet how can I limit access to leads/contact by City for example (or any other kind of field). I now I can hide full rows containing sensitive data, but I do not know whether I can sort based on what data a row contains, for example I want to have the London-based member of my company see only the contacts from London, not any other city,
Maybe the best way is to follow that strictly, first just get it working, even though it doesn’t match your requirements; and then try to change it to fit your needs.