Which HMAC Secret is used for "Register POS" requests?

According to the documentation, either channelLink or locationId may be used for calculating HMAC: usually channelLink ("present in most calls to partner endpoints") and otherwise locationId ("applicable to Dispatch API integrations and Get Products webhook for POS integrations").

However, there is only locationId in a "Register POS" request. Is this the field Deliverect uses for calculating HMAC for "Register POS" requests?

Also, is it channelLink which is used for "Order Notification" requests?