SAP V1, V2 and V3 udates

Document update is where the transaction (Actions) are updated in the application tables. this update is normally a synchronous update, I. e. if the update does not go through for whaterever reason, the complete transaction is rolled back.
Statistical update is the update of statistics for the transaction-like Lis or extractors for BW.

V1-synchronous update. if the update is set to V1, then all tables are update and if any one fails, all are rolled back. done for all transaction plus critical statistics like credit management, etc.

V2-asynchronous Update-transactions are updated and statistical updates are done when the processesor has free resources. if the statistical update fails, the transaction wocould have still gone through and these failures have to be addressed separately.

V3-Batch Update-statistics are updated using a batch (periodic) job like every hour or end of the day. Failure behaviour is same as V2 updates.

Statistical update is also used as to describe the initial setup of the statistical tables for Lo/LIS. when old transactions are updated in Lo/LIS as a one time exercise, then it is called a statistical update also. once these tables are upto date will all transactions, then every transaction is updated in them using V1, V2 or V3.