What does mean PPM unit and why it is the unit of flow rate for the ASU(Air start unit)?
PPM stands for Pounds Per Minute (not to be confused with Parts Per Million, which also uses PPM), which is a unit of mass flow rate.

The specification gives the mass flow rate rather than the volumetric flow rate because the mass of air going in and out of the ASU is the same, where the volume going in and out is different because of the compression.