One point that isn’t clear in the spec is what to to if the type doesn’t exist.
For example it’s defined for include: “If a server is unable to identify a relationship path or does not support
inclusion of resources from a path, it MUST respond with 400 Bad Request.”
But it’s not defined what to do for invalid types in “fields” (and the same is missing for unsupported “page”, “sort” and “filter” values or if the server doesn’t support one of theses features).