is a tool that splits LAS or LAZ files into multiple files based on some criteria. By default it splits the points into separate files based on the ‘point source ID’ field that usually contains the flightline ID. Others options are

-by_classification
-by_gps_time_interval 10
-by_intensity_interval 32
-by_x_interval 100
-by_y_interval 100
-by_z_interval 100
-by_scan_angle_interval 5
-by_user_data_interval 16
-split 1000000

The last of these simply splits the input into chunks of 1000000 points. One particular interesting splitting option is

-recover_flightlines_interval 5

That can be used to recover the original flightlines in, for example, LiDAR tiles that do not have flightline information (i.e. where the ‘point source ID’ is not populated correctly). For more details and other options not mentioned here see the README file.