This is an error where the server explicitly told the client to back off, such as a 429 or 503 Http error.
This is a server error that isn't explicitly throttling but is considered by the client to be something that should be retried.
Doesn't count against any budgets. This could be something like a 401 challenge in Http.
This is a connection level error such as a socket timeout, socket connect error, tls negotiation timeout etc... Typically these should never be applied for non-idempotent request types since in this scenario, it's impossible to know whether the operation had a side effect on the server.