A user coordinate system (UCS) is an origin and three vectors (X, Y and Z) that determine a position and orientation in 3-dimensional space. You can use an unlimited number of these to conveniently model your part.
One particular UCS is associated with a setup. A setup is an orientation and part program zero for a physical setup on the machine tool. The orientation and program zero are determined by the associated UCS and the setup contains additional information like the fixture ID and the name of the NC program that will be generated.
If setups are created directly by aligning with the stock, special UCSs are created with the string, UCS, appended to the setup name. For example a UCS called UCS_setup2 is automatically created for Setup2. These UCSs are used to store the location/orientation information for the setup. They cannot be deleted as long as their setup exists.