A control group is a random sample of your list that is excluded from the change you are testing. This enables you to compare the behavior of the test group versus the control group to determine the precise effect of your change.

This might be the group that gets your usual subject line A, against which you are testing subject line B.

Frequency testing
If you want to find out whether sending more frequently would be more profitable over the long-term, or whether it would wear down recipients with too many messages, you can compare the response of your test versus control group over a period of months to get your answer.

Next: What you can learn