What is "Window Scaling" (RFC-1323)?
The RWIN (TCP Receive Window) value in the initial TCP specifications was limited to 2^16 (or 65535, if you start with 0). With today's fast connections, this value has proven inadequate, however the 20-byte TCP header did not have spare room for larger numbers. RFC 1323 defines some "TCP Options" to overcome limitations to the original specs. Those TCP Options are in addition to the 20-byte TCP header.
Note: A reboot of the ProxySG is required for RFC1323 configuration to take effect.