The rules for how a display value are determined go like this:

• Is there a field with the display field set to true?
• Is there a dictionary override (for an extended table) with a display value override set
• Is there a field called number, name, u_number, or u_name

That is how the system determines what the display value is. It's not a query you can make via REST to say "Here's a sys_id of a user record, send me back the display value."

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