[Question] What's the idea behind ROLE_UNSPECIFIED ? and how should we use it ?

I’m trying to wrap my head around the roles defined in the current A2A protocol:

ROLE_AGENT
ROLE_USER
ROLE_UNSPECIFIED

From the docs, this is how I understand it so far:

ROLE_AGENT is used when an A2A server sends a response back to an A2A client
ROLE_USER is used when an A2A client sends a message to an A2A server

That leaves ROLE_UNSPECIFIED.

What’s the idea behind it, and how is it supposed to be used? I couldn’t find any explanation for it in the docs.

I’m also trying to map these roles internally when the A2A server delegates work to an OpenAI-compatible agent that processes the request in natural language. In that case, how would you recommend mapping A2A roles to the roles we’re used to in other APIs?

For example:

ROLE_AGENT → assistant
ROLE_USER → user
ROLE_UNSPECIFIED → ?
ROLE_TOOL or ROLE_SKILL (doesn't exists? those are implementation details? how can I represent a skill result in the conversation history?)

I understand that the A2A protocol doesn’t require an LLM to exist on the A2A server at all. That said, from my own testing it feels very practical - and fairly common - to have one there.

How are others approaching this in practice? Curious to hear your thoughts.