What is the main working principle of the Buchholz relay used in a three-phase transformer for internal protection?

In a power system protection scheme, the property that allows a relay to differentiate between a fault occurring within its own protected zone and a fault occurring elsewhere is known as:

Given the following statements: Assertion (A): Overcurrent protection is generally not used as the primary protection for alternators. Reason (R): Alternators have large short-circuit currents that can be sustained for long durations without damage. Which of the following is correct?