SBEF protection is connected to neutral CT of the transformer and works as standby protection to the earth faults downstream as well as for the incomer cable and transformer secondary winding earth faults.
REF protection is unit protection for a set of windings (primary or secondary, depending on CT locations).
"Standby" implies that the protection is not in service at all times, but standing by, ready to be implemented when needed. But what tells the standby function to go into service?
For example: in some line differential schemes, the distance protection is placed in standby and is only activated if the differential communications fail.
Does this STANDBY function have an initiating or enabling input?
Maybe a better description is "Backup" protection that is set to trip if the downstream functions fail to act quickly. Backup protection is always enabled.
Standby earth fault protection is as RRaghunath described. It is not normally switched in or out of service. Just another example how the same words can mean different things to different people. Broken delta/open delta are other fine examples.
SBEF protection is sometimes staged, for example if you have two transformers operating in parallel feeding a busbar equipped with a bus section (coupler) C/B, SBEF stage 1 trips the bus section C/B, stage 2 trips the transformer.
