Infos about .dat file and correlation between csv and .dat MAVIC AIR 2

I was looking at .dat file and I did a query (searching for VelN) and I got these columns*->* [‘IMU_ATTI(0):velN’, ‘IMUCalcs(0):velN:C’, ‘IMUCalcs(0):diffVelN:C’, ‘IMU_ATTI(1):velN’, ‘IMUCalcs(1):velN:C’, ‘IMUCalcs(1):diffVelN:C’]
But only IMU_ATTI(0):velN and IMU_ATTI(1):velN have number values. The other columns contain only NaN .
What is the different between (0) and (1)?

Another question is concerning the correlatation between .dat and .txt files.
I would like to merge .dat feature together with .txt ones (flight by flight)? I was thinking to merge them using timestamp but the .dat file only has “clock”. Moreover there are more rows into .dat file compared to .txt. For getting the .dat file related to a flight I created a script which extracts only value of motors > 0 and I split into different files. Doing so I can easily extract .dat features related for each flight.
Any suggesion is very useful! Thank you