Recognising raqabah as a noun is not a complete legal analysis. One must also identify the chapter of law, the situation creating the duty, and the object or act being qualified.
Expiation for an oath and expiation for accidental killing involve different contexts. Their texts illustrate unrestricted and restricted wording, but a final legal ruling requires bringing the relevant evidence together.
This prevents a linguistic classification exercise from being mistaken for a complete fatwa.