TT2A should be used when:

The use of TT2A, or the Treatment Tables 2A, is specifically indicated in situations where pain is relieved at depths greater than 66 feet. This is relevant during decompression incidents, such as those related to diving where a diver may experience decompression sickness or gas embolism.

When pain relief is noted at these depths, it suggests that the affected diver may be experiencing early signs of decompression sickness, which often necessitates careful management through the use of hyperbaric oxygen therapy protocols like TT2A. The parameters set out in TT2A aim to optimize the delivery of oxygen at appropriate depths to ensure effective treatment while minimizing further risks to the diver's health.

The rationale for the approach taken in TT2A considers the physiological responses under pressure and the necessity of controlled breathing gas delivery, making option C the best choice for the correct answer regarding when TT2A should be implemented.