How to have a custom field editable only by import and script

I would like to have a custom field that is editable only by CSV Import and SuiteScript. For UI, the field should be read-only.

I created a custom text field for Sale with these attributes:

Display: NORMAL
DEFAULT ACCESS LEVEL: EDIT
Default Level for Search/Reporting: EDIT

Then on a "customized Sales Order" form itself, I've tried setting the field with the Display mode "Disabled" or "Inline Text".

Attempts of populating the field via CSV Import do NOT update the field even though the CSV Import result reports success.

I've also tried importing via the "STANDARD Sales Order" form in which the custom field's Display mode is set to "Normal" as a workaround. CSV Import also reports successful imports, but the field is still NOT updated.