The Item ID relates to the ID the item has in the catalog. If you do not have a catalog and you are only providing a few services, you can number those services yourself.

When you enter the item ID, it will automatically fill out the line with the text you provided the first time you used that item ID.
Please note that the item ID field is not mandatory if you are sending to a Tradeshift contact. If you are sending to a NemHandel connection (EasyTrade), the item ID is mandatory.