Ok, documentation is a bit poor. X-MEN-Signature is actually a signature as explained in How to write a custom client interfacing a Mender server. Documentation could do with an update.
My confusion now comes from all the different APIs being able to send custom data. That article says we need to call Device inventory with a minimum of device_type and the artifact_name. But this isn’t mentioned in the API spec? Then we can call Deployments v2 but this also includes additionalProperties. If I just add additionalProperties here I get 204 No Content so I assume I need to declare all additional properties in the inventory. But if this is the case why do I have the ability to add them to the Check Update call? Is this in case the values change?