About Coordinate Systems
A model has following coordinate systems:
• WCS (world coordinate system)—The default coordinate system in Creo Simulate, which is a Cartesian coordinate system with its origin at 0,0,0.
• UCS (user coordinate system)—A coordinate system you create. A UCS can be Cartesian, cylindrical, or spherical. You can make a UCS the current coordinate system in place of the WCS. Creo Simulate associates user coordinate systems with the current body.
• Nodal coordinate system—The coordinate system of each of the nodes of the component or assembly used for orientation. The selected nodes are oriented for nodal displacement.
You can create a coordinate system using > 
Coordinate System. If you want to create the coordinate system for an entire assembly, be sure to select assembly datum geometry as coordinate system references.
Coordinate System. If you want to create the coordinate system for an entire assembly, be sure to select assembly datum geometry as coordinate system references.In either case, you may want to make the new coordinate system current if you plan to use it as a reference for multiple modeling entities.
Also see Axis and Component Equivalents in Different Coordinate Systems and Coordinate System Guidelines for more information.