All information taken from Christaian Quiennic's, Lisp In Small Pieces, 1994 edition.

Universal Global Environment

Reference	x
Value	x
Modification	(set! x ...)
Extension	no
Definition	no, define == set!

Frozen Global Environment

Reference	x
Value	x, but x must exist
Modification	(set! x ...), but x must exist
Extension	define (only one time)
Definition	no

Automatically Extendable Global Environment

Reference	x
Value	x
Modification	(set! x ...)
Extension	(set! x ...)
Definition	no, define == set!

Hyperstatic Global Environment

Reference	x, but x must exist
Value	x, but x must exist
Modification	(set! x ...), but x must exist
Extension	define
Definition	no