Hi Mohamed!

One is under the Sales section and the other under the Purchase section.

This happens when you set your contact both as a customer and a vendor.
Thus, the Payment Terms under the Sales section will be used for the customer invoices' you issue to your contact, and the one under the Purchase section will be used as the default one when you record a vendor bill from this contact.

It is less common to use Payment Terms in the Purchase section since, most of the time, you simply have to indicate the due date when you encode the Vendor Bill.

I hope this helps.

Cheers,

Jonathan