The Rcpt. by Item Function works in basically the same way as the Receipt Function with the difference being that you do not have to enter in the Document No you want to process. When using this Function you should enter in the Item No you want to receive instead.
Once the Item No has been entered, you will then be presented with a list of all the Purchase Orders which currently have that Item outstanding. You can click on the Arrow button at the top right in order to view some more information if relating to the Purchase Order if required.
You can then select the required one from the list and then proceed through in the same way described against the Receipt Function to process the receipt.