When I export selected packets using K12 (.txt) there is a header for each packet that looks like this "16:56:41,730,829 ETHER". What do the numbers represent?

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I am importing the txt file into excel where I can parse it and verify that the message is consistent with the Interface control document (ICD). I need to be able to track which packet is being parsed either by sequence number or time.

You probably want one of the options available under "Export Packet Dissections." If you want to track the sequence number or the time, then one way that is compatible with spreadsheets like Excel is to add all the relevant columns you need (such as sequence number, time, and payload), and export as CSV.

then one way that is compatible with spreadsheets like Excel is to add all the relevant columns you need

If somebody hasn't already filed an enhancement-request issue to allow exporting named fields, rather than columns, as a CSV file, so that the "add the relevant columns" field doesn't have to be done in this case, that needs to be done. (This can currently be done with TShark and the -T fields., -e {fieldname}, and -E separator=, options.)