Which is quieter but slower (i.e. less powerful), hence does not have the excess energy needed to cycle the action.
Many times a subsonic round turns a semiautomatic weapon into a single shot weapon, the price of the suppressed shot.
This will also light up our buddies at the BATF&E - that a suppressed weapon was used in a crime.
The armed security officers in my hospitals ER use subsonic rounds.
Can’t have 9mm rounds going through a wall and hitting an oxygen line.