In the MS-DOS Shell:
  You can get generic help by entering help at the
  command prompt.  You can get help on a particular command by
  entering
  help
command  at the command
  prompt (where command is replaced by the name
  of the command you are intersted in).
  
WWW Pages: There is a lot of information available on the WWW, including:
Using Environment Variables in Batch FilesCopyright 2019