Texas Instruments TravelMate 5000 Series User Reference Manual page 228

Memory Device Drivers
Some applications may require "backfill" memory, which is
the unused area of standard memory that can be used by
as Expanded memory. For example, an
EMM386.EXE
application may require only 256 KB or 512 KB of standard
memory, leaving 384 KB and 128 KB of backfill memory
space, respectively, for use as Expanded memory.
Configuring Memory E-7