So, a law in France prevents POS (point of sale) software from allowing users to modify or delete transactions and other data. To make sure they don't, software needs to be certified.

For three years (2016–2019), open-source software couldn't be certified, but since 2019, they consider any 'major modifications' of the software by any user, including the end-user, a reason to certify that forked software. So you can use and modify open-source software for your POS, with that condition if you want to use it for professional reasons. (though I have no idea how it's enforced)