SAP CRM Sales cycle flow is as below.
> Activity Scheduling>Activity >Opportunity Management> Quotation -> Sales Order --> Delivery (is in ECC) --> Billing (either in ECC or CRM)
Functionality From Visit Planning to Opportunity Management is present exclusively in SAP CRM. Quotation, Sales Order & Billing is present in SAP CRM as well as SAP ECC.
An invoice is a commercial document issued by a seller to a buyer, indicating the products, quantities and agreed prices for products or services that the Seller has already provided the Buyer with. An invoice indicates that, unless paid in advance, payment is due by the buyer to the seller, according to the agreed terms.
A billing document is requesting payment for goods previously supplied to the buyer.
Depending upon the nature of business an invoice or bill is generated.