How to lookup No by Type on SalesLine

A simple question: What’s the trick on i.e. Sales Line (T#37) to actually launch the different list forms. If you look at the field “No.” in the table its properties relates to various tables based on the “Type” field. But if you look at the “Sales Order” sub-form and the text box “No.” the “Look Up” property is set to “No” – so where is it determined which list form to run?