I believe that this particular file deserves special mention, because without it you will find it impossible to decrypt guest and manager data.
The file encryption_key.class.php was introduced in the middle of 2018.
It's purpose is to allow the system to encrypt and decrypt Personably Identifyable Information, I.E. guest and property manager's sensitive information.
If the file does not exist, on installation it is immediately created and put into the Jomres root directory, e.g. public_html/jomres. You can move this file, indeed it is recommended that you move it outside of the web server's root altogether and modify /jomres/configuration.php and set the $jrConfig['secret_key_location'] value to the absolute path of it's location.
Because this file is vital to decrypting your guest and manager data, you must not modify it and you must keep it safe at all times. Without it it's not possible to decrypt said data.