Below is the standard sales and distribution(SD) document flow in SAP .

INQUIRY

VA11 is the T-Code for inquiry and related tables are VBAK, VBAP.Once inquiry is done customer requests quotation.

QUOTATION

T-code for quotation in VA21, tables are VBAK, VBAP.After quotation flow goes for purchase order.

Purchase order t-code is ME21N, related tables are EKKO, EKPO.Based on purchase order sales order will be created.

Sales order t-code is VA01, related tables are VBAK, VBAP. Once sales order is created, delivery will be done.

T-Code for delivery order is VL01N, tables are LIKP, LIPS. Next the goods should be delivered through shipping

Shipping t-code is VT01, related tables are VTTK, VTTP.Once goods are shipped, go for billing.

Billing t-code is VF01, tables are VBRK,VBRP. Every bill has invoice...next provide invoice.

Invoice t-code is VF21, related tables are VBRK, VBRP.Invoice end of sales flow.