The environment variable is a value that you can use to retrieve the cpu version, computername and many other pieces of information on the server where your inMemory job is executed.
One use case could be to retrieve the cpu version, and then create a flow that acts differently depending on how many cores are available in your CPU.
The Environment Variable is placed in the toolbox in the lefthand side of your ETL Studio projekt.
Before you can use it directly in your project, you need to create a connection (data source):
You just click finish after naming your connection:
Now you can drag in your environment variable from the toolbox to your flow.
When you click the box that you just dragged in, you get the option to work with properties in the upper right corner - make sure you pick your newly created data source and name the container (in this case named processor).
Back when you double click your container in the flow, you get the option to pick the environment variables you need for your flow:
Here's an example where a few variables has been picked:
After running this job - the resulting table in InMemory Query tool showed this result:
Now you have some data on the environment inside your ETL job, and can create logic based on this information.