Only reading

A text field shown but locked: the editor sees the value and cannot change it.

For something filled in by the system — a computed code, a value carried over from an import, an identifier that must stay put.

Locked in the form, not in the record

The field is read-only in the manager. A write through the REST API, a seed or an import changes it as freely as any other. Treat it as a hint to the editor, not as a guarantee.

For a value nobody needs to see at all, use Hidden; for one that should be computed rather than stored by hand, use Status.