One example of a controlled group is a Parent-subsidiary controlled group: The simplest form of a parent-subsidiary controlled group consists of one business (the parent) that owns at least 80% of another business (the subsidiary).
The affiliated service group (ASG) rules were added to the Code in order to close several loopholes that practitioners found in the controlled group rules. Even if two businesses do not constitute a controlled group, the ASG rules will aggregate the businesses if they are sufficiently tied together in performing services for one another or for third parties.