iOS App specific vs. Master shared secret for refunds notification

If I have generated a master shared secret and an app-specific shared secret for a non-consumable in-app purchase, which one would Apple send in the Refund notification?

From the documentation, https://developer.apple.com/documentation/appstoreservernotifications/responsebody

password (string) The same value as the shared secret you submit in the password field of the requestBody when validating receipts.

The request body documentation says that

password (Required) Your app’s shared secret, which is a hexadecimal string.

It's not clear which one of the shared secrets Apple would send.

And, the refunds notification testing is not supported in sandbox, so can't really test it.

1. Does the app-specific secret takes precedence over the master one?
2. Can you choose which shared secret to use for App Store callbacks?