The AL developer preview is still evolving with frequent updates. Follow us on the Dynamics NAV Team Blog for the announcements.
Field No. Property
Sets a unique numeric ID for the field.
No two fields can have the same Field No. within a table. The Field No. is used for reference purposes and you can use the Name Property in code. The Name is automatically converted to the Field No. when AL code is compiled.
Usually, the Field No. is automatically assigned by Table Designer when you add fields to a table.