Fields shown in Mobile App

In the dedicated NAV app (Using NAV 2018) I wonder what denotes which fields are shown in the pages.

I have a field on a page, with a Table Relation to the Item Category Code table.  When I look up into the field, I would have expected the "Code" field to be at the far left.  However what I see is only Weight, Height and a couple of other custom fields added.  Where/how is it set for which fields are shown?  I thought it might be the field groups in the table, but no luck there, also played with the new ApplicationArea property of the Code field in the page but again no luck.

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