What scale is used to find the severity in this drug-drug interaction checker?

Copied directly from a drug interaction checker results page:

Drug Interaction Classification.
These classifications are only a guideline. The relevance of a particular drug interaction to a specific individual is difficult to determine. Always consult your healthcare provider before starting or stopping any medication.

Major: Highly clinically significant. Avoid combinations; the risk of the interaction outweighs the benefit.

Moderate: Moderately clinically significant. Usually avoid combinations; use it only under special circumstances.

Minor: Minimally clinically significant. Minimize risk; assess risk and consider an alternative drug, take steps to circumvent the interaction risk and/or institute a monitoring plan.

Unknown: No interaction information available.