When do we use set displacement/ set reference, please put an example in explaining

Karthic,

• Reference is the System in which the nodal coordinates of your grids are written;
• Displacement is the System in which the output (nodal displacement) are evaluated; 

For OptiStruct the displacement system is thus the one w.r.t. which you are also constraining or enforcing a displacement using SPC or SPCD respectively.

Hope this clarifies.

Thanks

Michele