The variable name is written in all-uppercase under DOS and OS/2.
Under Windows, which also supports lowercase environment variable names, the variable name is defined as ComSpec in the environment block, but as COMSPEC inside the DOS emulator NTVDM.
The environment variable by default points to the full path of the command line interpreter.
The COMMAND.COM file is copied to the ram disk during boot and the COMSPEC environment variable is set to the new location on the ram disk.
This way the boot disk can be removed without the need to reinsert it after a big application has been stopped.