In an explicit proxy configuration, the client (browser) is explicitly configured to use a proxy server, meaning the browser knows that all requests will go through a proxy. The browser is given the IP address and port number of the proxy service (the ProxySG). You could also use a Proxy Auto-Configuration (PAC) file to configure the browser to download the proxy settings from a Web server. When a user makes a request, the browser connects to the proxy service and sends the request. The disadvantage to explicit proxy is that each desktop must be properly configured to use the proxy, which might not be feasible in a large organization.
When transparent proxy is enabled, the client (browser) does not know the traffic is being processed by a proxy other than the origin server. To enable the ProxySG to intercept traffic sent to it, you must create a service and define it as transparent. The service is configured to intercept traffic for a specified port, or for all IP addresses on that port. A transparent HTTP proxy, for example, typically intercepts all traffic on port 80. To make sure that the appropriate traffic is directed to the ProxySG, deploy hardware such as a Layer-4 switch or a WCCP router, or the ProxySG's software bridge that can redirect selected traffic to the appliance. Traffic redirection is managed through polices you create on the redirection device.