Real-Time Query is a feature that allows queries to be run on a physical standby database while it is applying redo data. The relevant truths about it are:
Setting standby_max_data_delay=0 requires synchronous redo transport (A): For the real-time apply feature to function with no data delay (zero delay), synchronous redo transport must be used. This setting ensures that the data on the standby database is as current as possible before queries are executed against it.
Disabling Real-Time Query prevents the automatic start of redo apply when a physical standby database is opened read-only (C): If Real-Time Query is disabled, opening the standby database in read-only mode will not start the redo apply process automatically. Redo apply needs to be manually started to synchronize the standby database with the primary.
Reference:
Oracle Data Guard Concepts and Administration Guide