Coupling operates in the nodal coordinate system of each node coupled. It is good practice to keep your nodal coordinate systems consistent.

Degrees of freedom are coupled within a set but are not coupled between sets. You must not allow a degree of freedom to appear in more than one coupled set.

Do not include "grounded" degrees of freedom (having values specified via D or other constraint commands) in the coupled set.

In a structural analysis, coupling degrees of freedom to create a rigid region can sometimes cause apparent violations of equilibrium. A set of coupled nodes which are not coincident or are not in line with the coupled displacement direction may produce an applied moment that will not appear in the reaction forces.