Understanding the kettle.properties file

The kettle.properties file is a file created by Spoon the first time you run the tool. The purpose of the file is to contain variable definitions with a broad scope: Java Virtual Machine.

Why should we define a variable in this file? If we want to use a variable in several jobs and/or transformations, and the values don't change from one Job/Transformation to another, then defining the variable in the kettle.properties file is a good idea. Now you may guess why we put the SMTP related variables in this file. It's highly probable that you reuse them in many other jobs. Also, there are low chances that the values change.
In the kettle.properties file, you may define as many variables as you want. The only thing you have to keep in mind is that those variables will be available inside Spoon only after you restart it.

You also have the possibility of editing the kettle.properties file from Spoon. You do it from the main menu, Edit | Edit the kettle.properties file. If you use this option to modify a variable, the value will be available immediately.

If you defined several variables in the kettle.properties file and you don't want your file be modified by the tool, it's not a good idea to edit it from Spoon.

When you edit the kettle.properties file from Spoon, PDI does not respect the order of the lines you had in the file, and it also adds to the file a lot of pre-defined variables. So, if you want to take control over the look and feel of your file, you shouldn't use this option.