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The user-initial-environment is where the top-level
read-eval-print (REP) loop evaluates expressions and binds
definitions. It is a child of system-global-environment, which
is where all of the Scheme system definitions are bound. All of the
bindings in system-global-environment are available when the
current environment is user-initial-environment. However, any
new bindings that you create in the REP loop (with
define forms or by loading files containing define forms)
occur in user-initial-environment.
The variable system-global-environment is bound to the
distinguished environment that’s the ancestor of most other environments
(except for those created by make-root-top-level-environment).
It is the parent environment of user-initial-environment.
Primitives, system procedures, and most syntactic keywords are bound
(and sometimes closed) in this environment.
The variable user-initial-environment is bound to the default
environment in which typed expressions are evaluated by the top-level
REP loop.
Although all bindings in system-global-environment are visible to
the REP loop, definitions that are typed at, or loaded by, the
REP loop occur in the user-initial-environment. This
is partly a safety measure: if you enter a definition that happens to
have the same name as a critical system procedure, your definition will
be visible only to the procedures you define in the
user-initial-environment; the MIT/GNU Scheme system procedures, which
are defined in system-global-environment, will continue to see
the original definition.