in section 4.3. Sending Non-REGISTER Requests of outbound rfc 5626:

If the UAC is using a Globally Routable UA URI (GRUU) [RFC5627] that was instantiated using a Contact header field value that included an "ob" parameter, the UAC sends the request over the flow used for registration, and subsequent requests will arrive over that same flow.

what does 'that was instantiated using a Contact header field value that included an "ob" parameter' mean? which request the referred Contact header belongs to?

-- juha