The Supplier field in the detail pane has to do with joint checks.
In the construction industry especially, you will have a vendor with whom you have a contract to work on a project. This vendor, in turns, purchases materials or has a subcontract with a 2nd tier vendor or supplier. Even though the 2nd tier vendor or supplier is not a direct vendor, a payment paid to your vendor may have to include their 2nd tier vendor or supplier. This means you would issue payment to both your vendor and their 2nd tier vendor or supplier through a joint check. The vendor identified in the Supplier field is the 2nd tier vendor.
There is a lookup associated with the Supplier field. This is not a list of suppliers; rather, this is a list of vendors.
Viewpoint's Supplier field is located on the AP Unapproved Invoice Entry window
