Explanation
As mentioned in the question, the security appliances are using syslog to forward the logs to a central log aggregation solution. According to the web search results, syslog is a protocol that runs over UDP port 514 by default, or TCP port 6514 for secure and reliable transport1. However, some implementations of syslog can also use TCP port 514 for non-secure transport2. Therefore, to allow the web servers to connect to the central log collector using syslog over TCP, the firewall rule should allow TCP 514 outbound from the web servers to the log collector.