what's different between RACK_ENV and RAILS_ENV?

You can use RACK_ENV as well as RAILS_ENV except for RAILS_ENV is prior to RACK_ENV.

rails/rails.rb at b0b4b176b0e061a4f03ddce669637b7d6c37aa33 · rails/rails

Rails applications uses RAILS_ENV. Other Rack-based applications use RACK_ENV. If you have a Rails application, ignore RACK_ENV.

EDIT: The other answer is more correct.