Under Registry - Items - Units enter in the various units you want to use to describe your items in your invoices.
Enter new units by typing a code and a description in respective columns. Click on Save. Delete a unit by clicking on the bin icon at the end of the row.
If you create invoices in different languages, you must enter translations of your units by clicking on EN between code and description. A pop-up window will appear where you can enter the translations. Click on Save when you are done. See below.