Here is the definition of the network contribution time that is in the documentation: The estimated time spent to transfer resources with an immediate impact on the user action response time.
I would believe that it the time spent by within the transaction for requests and responses to be transferred between the client and the backend servers.
The network contribution is estimated by checking the speed of the user's network connection (via bandwidth detection which must be enabled in UEM) in combination with request/response sizes (must be enabled in the web server and servlets sensor configurations). So in that sense it is the network contribution from the client perspective as David has said.
And for clarification, the user action response time is solely based on when the onload event completes in the browser, it does not try to add up server side + client side or anything like that.