I believe the cid is the Log Collector ID number. It should match up with the internal Netwitness ID number that represents the log collector that the log came in on. It is unlikely that you have 704 Netwitness devices in your environment however every time a device is removed and added back the ID numbers increment. My guess as to rn probably means the Remote session ID Number. This may represent the internal ID number of the log when it was collected by the Log Collector.
These are guesses based off my experience with Netwitness. If anyone else has a more definitive answer, please provide it.
The eid value (Execution ProcessID) is the pid of the process that ran the task,This is a decimal representation of the same information from the "Process ID: 0x2b8” (hexdecimal) section of your logs, so from an anlysts's perspective knowing the PID is valuable info, plus if there's ever a mismatch between these 2 numbers, that could be interesting.
For the other two, I don't know how an analyst might find them useful in an investigation.
The "rn=" may not be interesting to an analyst during investigation but from a log collection validation standpoint it can be used to determine if you are getting all the logs from the Windows system that you are collecting from or if you are collecting Windows logs in two different locations. For example, if you are collecting from a single server from two different log collection systems, you can use the "rn=" to reconcile the events between the two systems to make sure there are no missing events in the collections.