With UDP and TCP protocols, you can choose a local port number or a remote port number. When you choose ISO-on-TCP.
Protocol, the port number is fixed as 102.
Port 102 is dedicated to the ISO-on-TCP protocol. You cannot set the port number for this protocol, but you can set TSAP for local or remote partners.
Detailed reference manual: S7-200 Intelligent System Manual V2.2 Page 435 8.4.7.3 Port and TSAP.