What is the difference between V1 and V2 update?

In SAP, the update process will be performed in several steps. The time-critical part of an update is called a V1 update, and the non-time-critical part is called as V2 update.

The V1 update will be handled by the UPD work processes (UPD: Update process for making V1 (time-critical) database changes) and the V2 will be handled by the UP2 work process (UP2: Update process for executing V2 (not time-critical) database changes).

To view the status of these updates, use transaction SM13. However, if you see any failed updates, find out the root cause and advise the users to perform the task again rather than posting it back from SM13.