Buyers Setup Employee ID
The buyer's setup page has been modified either during one of the last two Rollup patches in 12.1 to include the Employee ID. I am not sure which patch added the field. This change is also in 12.2.
The Employee ID field is a free entry field, when adding a new record. During a new record creation, a null value will populate the employee ID after the record has been saved. After the record is created the field can be edited and the entry can be changed. There doesn't seem to be any dependencies on the information and looks like the purpose might be used to identify two buyers with the same name (there is an open bug with this request).