BOOTOS2 is a program to create OS/2 boot diskettes or an OS/2 maintenance partition.
It is mentioned in this document because it contains a tool named BOS2REXX.EXE. BOS2REXX.EXE is a little program to load the REXX support. This is necessary if OS/2 was booted from diskette (see Using REXX if booted from diskette).
In contrast to the program SRVREXX.EXE (which is part of the CID package from IBM) you can use REXXUTIL-functions in REXX programs if you're using BOS2REXX to init the REXX support (see Using REXXUTIL).
Another useful tool in the BOOTOS2 package is BOS2SHL.EXE. This program replaces the IBM program SYSINST1.EXE. In contrast to SYSINST1.EXE, BOS2SHL.EXE supports the START command so you can have multiple OS/2 fullscreen sessions even if you've booted from diskette. BOS2SHL.EXE also supports the use of the file STARTUP.CMD.