RE: Item No. Code descriptions?
Hi
Code is a manual entry, just an option you can use: manual entry, no real meaning I would say.
Maybe 1 important point here related to the comments process
"you can specify if you want comments to be transferred from one type of document to another resulting type of document, such as from a sales order to a sales invoice. You do this in the Sales & Receivables and the Purchases & Payables pages respectively."

Next to comments, you can use extended item text feature, not sure what is your purpose here.
Hope helpful