I understand the disconnect. The Custom field needs to be named in the reference type settings and has to be defined in every ref type that you intend to use the custom 1 field in.
For example, in the attached snip, I first chose to edit preferences, then went to the Reference Types, and selected “interview” ref type, and now I can go down to the field and add ID to the custom 1 field name. click okay. Now that field will appear whenever a record is defined as “interview” but not for other ref types.
In my Journal article ref type for example, custom 1 has already been used for “Legal Note” and custom 2 for the PMCID. Even if I display “custom 1” and name it ID, it will still show the contents of the custom 1 field in the general library column you renamed (even if it is a legal note). And it will still show the “legal notes” field in the right hand reference display.
If the field is undefined in a ref type, it will only show up if you filled it in some way (say if you filled it when it was defined as “legal note” in journal article and you then changed the ref type of the record to “interview” which has no defined field name for custom 1).
