The system heap is a special area of RAM set aside for things like fonts
and DAs. The heap zone is used mostly by operating system data structures.
The heap zone is an area in memory that may be allocated in relocatable and
non-relocatable pieces, called heap blocks or heap pieces. The heap zone
usually begins at $B00 and extends to $CAFF, but you should not count on
those addresses being correct.
If you ever see an error message with the number -108, it probably means
you need more space in your system heap. Increasing its size will help you
if you have many fonts and DAs.