It doesn't save anything, it is just a startup script to get your tmux session set up the way you want. You can however, have env vars in the script or source from a particular file. e.g.

  session "grnc_hub"
  directory "/home/minhajuddin/a/grnc_hub"

  window "server", [
    pane("RAILS_ENV=production rails s"),
    pane("source ./.env && rails s"),
  ]
  ...