SIP INFO was designed to carry session related information during a session.
SIP was designed to setup and tear down sessions, with little thought given to what happens after the setup, but before the teardown.
SIP INFO (RFC2976) was designed to fill this gap. (Obsoleted by RFC6086)
It’s predominantly used now to carry DTMF info during a call.
The message body in this case contains the signal (DTMF digit 3) and the Duration (180ms).
The SIP Info standard doesn’t actually stipulate how the message body should be structured, but the above shows a defacto standard that’s now common, although not set in stone.