HP 48G Series Quick Start Manual page 49

Notice that there are several kinds of variables indicated in the
diagram:
System variables. These variables are hidden from you, but are used
and updated by the operating system to keep track of everything as
you work.
Local variables. These are temporary variables, created by a
program, that only exist during the execution of that program.
Global variables. These are variables you create (by assigning a
name to an object) and can edit, evaluate, and manipulate. They
are stored in user memory, where they are quickly and immediately
accessible. Global variables are hierarchically organized into
directories.
Port variables. These are like global variables except that you may
not edit them and they cannot be hierarchically organized. They
are stored in port memory, where they are in "long-term" storage.
The HP 48G model only has one port (Port 0), but the HP 48GX
model can be configured with as many as 32 additional ports (each
containing up to 128 KB of memory).
The remaining lessons in chapter 4 illustrate some of the common
things you can do with user memory and global variables. The
HP 48G Series User's Guide gives you more information about the
"boundaries" within SYSRAM and otherwise managing memory
(chapter 5) and how to use and access port memory (chapter 28).
Understanding and Using Memory 4-3