By default, the ProxySG performs load balancing using the round-robin method without giving preference to any ICAP server in particular. This is also the default behavior when ICAP Service Groups are configured when not using Malware Scanning for ICAP processing. This means that all the ICAP servers have a weight of 1, so the ProxySG evenly forwards requests sequentially to servers as defined within the Malware Scanning list. You can find more information on Service Groups load balancing in article TECH241125

Although this behavior can be changed when using Service Groups by using the weight setting, this cannot be changed when using ICAP servers through Malware Scanning.