This page describes atomicity and versioning of Aggregate modifications.
Event Batches
When you update an aggregate you do so by saving batches of events. The reason for this is to ensure atomicity in the case when a command results in multiple events being emitted simultaneously.
Aggregate versioning
All aggregates have a current version. Each batch of events that is saved to the aggregate increments the aggregate version by one, regardless of whether the batch contains a single or multiple events.