and what does it represent ?
The scan rate parameter in the config.ini file on your USB memory drive represents the rotation speed of the mirrors inside the scanner. While the PRR (or PRF) change the number of pulse emitted, the scan rate would change the spatial distribution of the points on the ground.
In order to get an homogenous distribution (usually the goal), you must adapt the scan rate to the flight height, the speed, and the PRR.
The scan rate for a regular point spacing can be calcuted using the table "YellowScan Voyager SCAN_RATE Calculator" provided with your unit.
If you can't find it, please send a request to YellowScan support form :
https://www.yellowscan.com/contact/#support