Flush Type Grease Fittings are for installation where protruding fittings won't work.  Flush fittings sit flush or countersunk, and are not damaged easily.  Flush Type Grease Fittings are referred to as Midget Grease Fittings, Flush Zerk Fittings, Flush Type Grease Zerks, Flush Type Drive Fittings or Flush Type Press-In Fittings.