Why is the relationships -> Contacts on a sales order not a selection of existing contacts of a customer? Can anyone explain the purpose of the contacts on the sales order when they don't seem to be connected to the customer record? Kind of useless if you ask me...

For sure, you can make a sublist of the contacts from the customer record based on entity matching, it's more useful.

@Castor Thanks, I think I know what you're saying but can you elaborate on how to do this?

Is the customer set on the Project record? NetSuite considers the Project and Customer the same record type; relationship. On the customer > sales tab > projects would be listed for quick review. The sublist view is customizable. The Project Name in list view will include the the customer in the project name in most list views. This can be separated depending on the field chosen on the report or saved search. Let me know if you'd like to connect and review your use case.