Chapter 6 discusses constraints, with a subsection on AUTOSPC, and Chapter 11 discusses Inertia Relief.
In a nutshell, AUTOSPC is intended to assist by automatically constraining singular DOF in the structure. Inertia Relief is used in Linear Statics to allow a structure that can undergo rigid body motion to run. Usually, you must have at least a statically determinate set of fixed boundary conditions (NO rigid body motion). Inertia Relief is a technique to allow a static solution even for a structure with rigid body motion.
Please review those sections of the Linear Static Analysis User Guide and the differences should become clear.
