2012

You can define any UCS by specifying an origin and one or more rotations
around the X, Y, or Z axis.
Z Axis
Aligns the UCS to a specified positive Z axis.
The UCS origin is moved to the first point and its positive Z axis passes through
the second point.
Object Aligns the Z axis tangent to the endpoint that is nearest to the specified
point. The positive Z axis points away from the object.
Apply
Applies the current UCS setting to a specified viewport or all active viewports
when other viewports have a different UCS saved in the viewport. (UCSVP
system variable).
Viewport Applies the current UCS to the specified viewport and ends the UCS
command.
UCS | 1971