pac file return a proxy without port number

Question

Yesterday, I received a pac file which defines only proxy but not port number, like this one:

function FindProxyForURL(url, host) {
  if (shExpMatch(host, "vpn.domain.com"))
  return "PROXY proxy.mydomain.com";
}
  

Then I used

ipconfig /displaydns
netstat -n 

cmds to test this pac file, found that, [proxy.mydomain.com] always uses port 80 in either HTTP or HTTPS.

My question is, why does it use port 80, is it some kind of default definition?