k6rot controls the stiffness given to the drilling degree of freedom in plates. Otherwise plates at 90 degrees to each other but sharing 2 corner nodes would have no connection for that degree of freedom. Autospc permits degrees of freedom that are zero or very small in comparision to their neighbors in the K matrix to be set to zero and removed from the solution (aka grounded and have no displacement). This relative cutoff is set using the maxratio parameter. MSC's linear handbook are very goo in this respect and are well worth reading.
Whatever you do do not go off and make any model without being very clear on these and several other basic concepts.