The displacement co-ordinate system is the co-ordinate system you have asked for the displacements to be written in. Normally they are written to the output file in the global cartesian co-ordinate system but in some codes you can ask them to be written in a cylindrical co-ordinate system, for example. Displacement plots are unaffected.
The displacement coordinate system is ALSO the system within which the boundaries and loads are operated. So for Sunny's example above, if one wanted to apply a boundary condition radially, a cylindrical coordinate system would be used to apply them.
This is an important addition, in that the earlier point does not actually change the global results; it merely describes the outputs in a different basis. In this example of applied boundary conditions or loads, the different coordinate system actually can affect the results.
Brad
Agree with Brad. Another very important application of a displacement coordinate system other than the global is when you want to apply coupled degrees of freedom between two mating parts that are tightly attached, for example pressed together by an external load, in a particular direction like r and theta. Now, if you model this without coupled defrees of freedom, lets use the cylinder example again, and assume two cylinders pressed against each other without any lubrication, there would be relative displacement between the two parts at the contact surface, which is not real. So, you need to apply the coupled d.o.f constraint on the nodes on both parts mating on the respective surfaces, in the r theta direction, the axial is taken care of by the contact definition. By default, these nodes have their displacement CS defined as global, so to be able to apply the particular constraint, you'd have to change their displacement CS to the radial one. Sunny.
