The "buyer's reference" field cannot be edited as the field is automatically updated by the system.
The main rule is that the field contains a reference to which document the 'document' you are looking at was created from. If you are on a purchase order response (SOA) that is made from a purchase order (SPO), the "buyer's reference" will contain for example SPO/123456.
The content of the field can be reference to a batch job if data has been loaded, in addition to automatic jobs or loads run in Rambase.
This applies to 'Sellers ref no' field on customer orders (COA) too.