SD and finance modules are integrated in SAP to automatically generate accounting documents for sales activities. When goods are dispatched, a material document is created which triggers a finance document to be generated, with the GL account and amount coming from OBYC settings. Similarly, when a billing document is released, the pricing procedure uses information like order type, customer, and sales area to select the appropriate procedure and determine the GL accounts and amounts to post to finance. This process of automatic accounting document generation from sales documents is known as SD-FI integration in SAP.