Companies are counted as having used a feature when a User of the Company initiates the Event associated with the feature.
For example, let's say your SaaS application is tracking FeatureA by whenever someone interacts with ButtonA. Let's also say UserA is part of CompanyA.
When UserA interacts with ButtonA, an event is sent to Bucket. Since UserA is associated with CompanyA, that company has now been counted as having interacted with FeatureA.