What signifies that appliances and equipment are to be isolated during a test?

The correct answer focuses on the use of blanks, blind flanges, or caps as a means to isolate appliances and equipment during a test. These physical barriers serve to block off any potential pathways for fluids, gases, or electrical currents, ensuring that the equipment in question is completely disconnected from its operational systems. This method is critical in testing scenarios, as it prevents any accidental activation or leakage that could pose safety risks and ensures that tests can be conducted safely and effectively.

Other methods mentioned, such as turning off power sources or labeling equipment as inactive, may not provide the same level of assurance as physically isolating the system with blanks or caps. Turning off power sources does reduce the risk of electrical hazards, but without physical barriers, there's still a possibility for equipment to be accidentally energized or to miscommunicate the status of the equipment. Labeling can aid in communication but does nothing to physically prevent the equipment from being used or activated. Therefore, the practice of using physical barriers is the most reliable approach when isolating appliances and equipment for safe testing.