At the beginning of our test suite, we have code to check the environment and set some project variables. The tests that need to know about the environment use those variables. For instance, one of the variables holds our UI version. Later versions have different buttons in the toolbar, so we can use that variable in a test to know which buttons are available for use.
I'm not sure what you're looking for in the drop-down. In our case, the variables are always the same type, just different content. You can always pick a Project variable or a Local variable for your parameters.
The variables can also be different in each different instance of the project that you save, so if you are putting a separate project on each machine you run, then you can set the variables manually and just leave them that way. That's more specific for each machine but more difficult to maintain.
