This dialog allows you to select some environment variable that you expect to find on the target machine.
In the "Name" text field enter the variable's name. If the variable is present on your system you can select it from the drop-down list.
If this variable doesn't exist on the target system it will be expanded into a null string.
You can choose whether to expand the environment variable's value at the moment when the application is being installed or to expand it every time it is run. The second choice is very useful if you expect an environment variable to change it's content after the installation is complete.