If the URL matches a pattern or a site that requires a proxy, it will connect to the proxy server rather than going directly to the site. Our PAC is configured so that only URLs requiring access from the uchicago.edu domain will use the proxy server. User starts Browser, connects to proxy server to get PAC file. Proxy server returns proxy.pac, which configures browser to do proxying. Browser retrieves URL directly.
Browser contacts proxy server and requests page.