The wall was first tested with a bubble level to see that it was as plumb as possible and to make sure that it was not made to slope inward to cheat on the dust test.   The dust test was to see if there were any bumps and surface irregularities in the plaster work.  The dust would stick to any area projecting out a little, and would not stick to a perfectly vertical wall surface.
    , the building inspector would come and shake a bag of chalk dust in the air and then seal the room.   24 hours later the room was opened and if there was any dust anywhere on the walls, it proved that the wall was not perfect 
  Please explain the above. I don't get the connection between chalk dust in the air and on the walls.
  Ralph