A Marker represents the coordinate system fixed on a Solid Body (Rigid Body), Nodal EasyFlex Body or Nodal FE Body. The coordinate system has a position and orientation and a marker is thererfore often used to measure position, velocity or acceleration at a point on the body or node with a Function Expression or User Subroutines. When the marker is defined on a Modal Flexible body or Modal EasyFlex body, the Motion Solver will return an error.