A building block (BB) event is an event that is used solely for enabling other events, dimensions, or session attributes.
Building block events are temporary and exist only in the context of event processing. With a building block event, you can track significant items in a session without incurring the recording costs of writing the event to the session record.
For example, suppose that you were interested in the elapsed time of a hit across the network. Since this value is detected at the individual hit level, saving values for every hit can be expensive, and you might be interested only in values that represent an exceptional use of network time. For such a scenario, it makes sense to create a building block event to capture high-volume events and then create an event or attribute to detect the exceptions.
Specifying an event to be a building block event
You can specify that an event is a building block event, by selecting Building block only when you create the event definition.