Nastran DOF Set Definitions

The Nastran Quick Reference Guide has a section describing the "Degree-of-Freedom Set Definitions."  Here's a brief but very useful description:

J - All DOF (6 DOF per GRID + 1 DOF per SPOINT)
G - Same as J, but matrices include contribution of upstream superelements
N - After elimination of dependent DOF from constraint equations (RBE, MPC, etc.)
F - After elimination of restraints (SPC and AutoSPC)
A - After elimination of omitted DOF (OMIT or ASET cards)