The User Coordinate System defines

• The XY plane, also called the work plane, on which objects are created and modified
• The horizontal and vertical directions used for features like Ortho mode, polar tracking,
  and object snap tracking
• The alignment and angle of the grid, hatch patterns, text, and dimension objects
• The origin and orientation for coordinate entry and absolute reference angles
• For 3D operations, the orientation of work planes, projection planes, and the Z axis for vertical direction and axis of rotation

You can change the location and orientation of the current UCS by clicking the UCS
icon and using its grips, or with the UCS command. Display options for the UCS icon
are available with the UCSICON command.

Understand the UCS in 3D

When you create or modify objects in a 3D environment, you can move and reorient
the UCS anywhere in 3D space to simplify your work. The UCS is useful for entering
coordinates, creating 3D objects on 2D work planes, and rotating objects in 3D.

NOTE: The UCS icon follows the traditional right-hand rule in determining positive axis
directions and rotation directions.

The UCS in Paper Space

You can move and rotate the UCS in paper space on a layout; however, the UCS in paper
space is restricted to 2D operations.

Understand the World Coordinate System (WCS)

All objects in a drawing are defined by their coordinates in the World Coordinate
System (WCS), which cannot be moved or rotated. The WCS and the UCS are initially
coincident in new drawings.