This configures the line-rate versus PLCP framing mode for the system's secondary DS3 clock port, the counterpart to cardSystemPrimaryClockModeConfig. This is the equivalent setting for the backup timing path, and an admin bringing a secondary DS3 timing reference online needs to independently verify this mode matches that link's actual framing, since it's configured separately from the primary clock's mode. For example, an admin who correctly set the primary DS3 reference to plcp-mode but forgot to also update cardSystemSecondaryClockModeConfig would find the secondary reference degraded even though the primary works fine.