A simple question for COMSOL v3.5a

Hello,

I am now trying to use the femlin() in COMSOL v3.5a. According to the documentation, [Ke,Le,null,ud] = femlin(fem);
Ke and Le are the stiffness matrix and load vector. null is the matrix represents the transformation between constraint and unconstraint system. But what is ud?

u = u0 + Ke/Le + ud. Is it the rigid body displacement?

Thanks

Dan