If you keep an external customer number on your Magento customers, enter the name of the (extension) attribute that holds it.
Its value is used as the external customer number (ID) when orders are synced.
For example fortnox_cust_no or sap_customer_id.
Leave it empty if you don't use one.
Good to know
Type the attribute exactly as it is named in Magento – same spelling and same upper/lower case.