The supplier reference number is a required field when connecting your accounting software with Bluechain. Without this information, the bill will not appear on the Bluechain platform.
If you have already synced your accounting software to Bluechain, you can simply add a supplier reference number to your bill in your accounting software and resync to Bluechain to ensure that it appears. This is the information that your supplier will see in their bank account when they receive the payment, so please ensure it's clear to them what the payment is for.