There are several ways to add items (products/services) to a quotation, job/order, etc.
Adding an Item from the List/Database
Use this button:
Adding a One-Off Item
These items exist only in the quote, job/order they have added to, as well as the subsequent documents derived from it (i.e. a delivery note, invoice, etc.).
Adding it later to the List/Database
An item that has been added as a one-off can then be added to the database list using the item menu -> Create/Update -> Details only. This brings up the new item edit screen.
Adding an Item to the Document and List/Database Simultaneously
Use this button to add a new item to the quote, job, order, etc., and the list/database simultaneously.