In the first step of setting up a new experiment in Compose, you will se the option to "Define Control." BY default you will see that it is named "Original" and is set to "Original Default." If you are testing what is currently on your live site versus a change you are making, then you can leave this as is. You do not need to edit this unless your control differs from your current live site.
IMPORTANT: This is NOT where you define the trigger for the experiment. During step 3, Targeting," you will have the ability to define when you would like the experiment to trigger. For example, if you are testing a change to your homepage, and your control variant will be the current live site, then you should leave the control variable set to "Original Default" and on the targeting section you will set the trigger as a page URL with your homepage URL listed. Example below.