This field relates to the above (Proxy) field, and if a proxy is being used, both must be set. To use an HTTP proxy you must not only supply the address of the proxy but also the port number to use for connecting to it.
Consult your carrier's recommended APN settings to see what port number you need to use if you are using an HTTP proxy. If you do not need to use an HTTP proxy this should remain empty.
Common port numbers for HTTP proxies may include 80, 3128, 8080, or any high number ending with the digits "80".