Supplier rules are set on a client basis. This means they will apply to everyone submitting to the client account, including both your clients themselves and your staff.
If you have set a rule for a particular supplier, all the items submitted for that supplier will be subject to the rule you have set, regardless of who has submitted it, or whose account it has been submitted under.
Once default categories are set up for each client, you don’t have to worry about anything else, each item a user submits to that client will be affected by the rules already set.
If a supplier is used by different employees in different ways, the best way to deal with this is to set a partially completed supplier rule, leaving out the type of information that changes (category for instance). This saves you time while being flexible in the same time.