This wiki explains how one can place any object on the set ACS plane using ACS Plane Lock option. This option when turned on, forces each data point to lie on the XY-plane of the active auxiliary coordinate system, setting all Z-coordinates to zero. This concept applies only to 3D files.
Please follow the below steps to achieve this.
1. Set the ACS where you want to place the objects on that ACS Plane.
2. Then go to the Drawing Aids > Locks > ACS Plane Lock and turn on this option.
3. Now. when you place any objects say smartline it will be drawn on the set ACE Plane only.