Transaction Guard provides benefits in terms of transaction consistency and recovery in a Data Guard environment:
It provides application continuity by rolling back uncommitted transactions interrupted by a failover or switchover (B): Transaction Guard ensures that any uncommitted transactions at the time of an outage are rolled back consistently, thus preserving the integrity of the application's data and state.
It protects against recoverable errors during a planned or an unplanned outage of a primary database (D): Transaction Guard offers protection against errors that can occur during outages, allowing applications to resume operations more quickly and reliably after recovery.
Reference:
Oracle Database High Availability Overview
Oracle Real Application Clusters Administration and Deployment Guide