The following methods are available:

3 points

Define a rectangular, cylindrical, or spherical coordinate system by
selecting the origin and, optionally, two additional points. In the case of a
rectangular system, the second point defines the
Xaxis, and the
X–Y plane passes through
the second and third points, as shown in
Figure 1.
This is the most versatile tool for creating a datum coordinate system, and you
should use it when possible. For detailed instructions, see
Creating a datum coordinate system defined by three points.

Offset from CSYS

Select a coordinate system; and provide the location of the rectangular,
cylindrical, or spherical datum coordinate system by specifying an offset, as
shown by the example involving a rectangular system in
Figure 2.
You can specify the offset by entering a value or by selecting a point. For
detailed instructions, see
Creating a datum coordinate system at an offset from another coordinate system.

2 lines

Select two edges that define the rectangular, cylindrical, or spherical
coordinate system. In the case of a rectangular system, the first edge defines
the Xaxis and the
X–Y plane passes through
the second edge, as shown in
Figure 3.
For detailed instructions, see
Creating a datum coordinate system defined by two lines.