We have a common requirement: when we manage organizations with multiple sites and warehouses, we need to define user access per site and warehouse. We made a major customization for past projects : a user can be assigned one site (or more) and when accessing a transaction, the user can only see his/her records filtered on the authorized site.
This is done on all transactions (PO, SO, inventory movement, etc.). We need only on headers (not lines like SO and PO).
the only standard solution is with Legal Entities but not good as we can have multiples sites / store / warehouses into one legal entity