This configures which physical port on this card, if any, should be used as the chassis's system secondary clock source, with 0 meaning no port is designated. Configuring this designates the chassis's fallback timing source, so an admin should verify this points to a genuinely independent port or reference from the primary; pointing both primary and secondary at the same physical feed defeats the purpose of having a secondary reference at all. For example, an admin discovers both cardSystemPrimaryClockPortConfig and cardSystemSecondaryClockPortConfig were mistakenly set to the same port, meaning a single link failure would take down both timing references simultaneously.