Some operations within the ev.energy API only make sense in the context of a singular user. For clients authenticated via the Authorisation Code grant type that context is inherent, but for clients which use Client Credentials, the token does not represent a single user.
In order for these endpoints to make sense, and to minimise varying behaviour between the two grant types, the API supports a custom header, EvEnergy-User which allows the client to provide that context. The header should be set to the URL of the user on which the operation should be performed.
Some endpoint require this header to be set and will return a 400 status code response if it is missing. The relevant API reference pages will make it clear when this is the case.