Doubt in sales and receivables setup

Hi, Can any one please explain me for what Shipment on Invoice and Return Receipt on Credit Memo fields are used in sales and receivables setup. Please explain me in detail iam in state of confusion.

From F1

Shipment on Invoice Field

If you place a check mark in this field, the program will automatically create a posted shipment, in addition to a posted invoice, when you post an invoice.

If you leave the field blank, the program will create only a posted invoice when you post.

Return Receipt on Credit Memo Field

If you place a check mark in this field, the program will automatically create a posted return receipt, in addition to a posted sales credit memo, when you post a credit memo.

If you leave the field blank, the program will create only a posted sales credit memo when you post.

Now where is the confusion ???

It means that when you post an invoice, the system will automatically create a posted shipment.