Draw another plane
@Anonymous wrote:
why only xy plane?
You can draw in other than the XY plane in various ways. They may have meant that when you draw by picking points with the cursor, you will always be drawing on the XY plane of the current User Coordinate System [not necessarily always the World Coordinate System's XY plane]. But you can still draw off of that plane if Object-snapping to something that has a non-zero Z coordinate, or you can specify Z coordinates in any designation of a point, using point filters or spelling out 3-coordinate points, or you can set the ELEVATION System Variable to a non-zero value.
Some things can be drawn using different Z coordinates for different parts of them [Lines, 3DPolylines, Splines], but some things are "locked" into the same Z coordinate as the first thing you specify [a Circle or Ellipse or Arc or non-3D Polyline or Text/Mtext will always lie in or parallel to the current XY plane, at the Z-coordinate level of the first point specified, etc.], but they can be above or below the current XY plane at a non-zero Z elevation if you choose.
Kent Cooper, AIA 