Port following is set when two services need to share persistence records. Typically this is done for HTTP and HTTPS services so users maintain a server session regardless of whether they connect securely. This is explained in the Port Following Feature Description document which can be found at here
Port following has several requirements:
- The Virtual Services must be on the same IP address
- The Virtual Services must have the same set of Real Servers
- The Virtual Services must be using the same persistence options
After meeting these conditions, in the Virtual Service modify screen there will be an option under Advanced Properties for Port Following. Be sure to set this on both services to ensure that port following is done bidirectionally.