Intercepting proxy and Forced proxy

Intercepting proxy :
An intercepting proxy (also known as a "transparent proxy") combines a proxy server with a gateway.Connections made by client browsers through the gateway are redirected through the proxy without client-side configuration.Intercepting proxies are commonly used in businesses to prevent avoidance of acceptable use policy, and to ease administrative burden, since no client browser configuration is required.It is often possible to detect the use of an intercepting proxy server by comparing the external IP address to the address seen by an external web server, or by examining the HTTP headers on the server side

Forced proxy:
The term "forced proxy" is ambiguous. It means both "intercepting proxy" -- because it filters all traffic on the only available gateway to the Internet .
and its exact opposite,non-intercepting proxy"-- because the user is forced to configure a proxy in order to access the Internet.
Forced proxy operation is sometimes necessary due to issues with the interception of TCP connections and HTTP.This is primarily because the client thinks it is talking to a server, and so request headers required by a proxy are unable to be distinguished from headers that may be required by an upstream server.