As said by Ricardo, in Classic and Extended Classic Scenario - "Goods Receipt" & "Invoice" can be created either in SRM or in ECC system. However the linkages are always maintained under the prior document.
As an example - A shopping cart can result in Purchase Order (Material Items), which is than followed by Goods receipt & Invoice. This means, a Invoice cannot be created directly to a shopping cart but there must be another legal document like Purchase Order created in between. If you are using Classic scenario, then the purchase order exists in your ERP system only. If you are using Extended Classic Scenario, then your purchase order exists in SRM as well as in ERP systems.