Duct and Envelope Tightness (DET) testing is mandated by the International Energy Conservation Code (IECC). These mandates set building construction and HVAC install standards as well as new testing and training requirements. Simply put, the days of up-sizing the a/c until it's cold inside are gone. The tighter a system, whether you're talking about just the a/c, or the house itself, the better things are going to perform, the more comfortable the inhabitants are going to be, and the less energy the home will consume.
The "Duct", in DET, refers to the testing diagnostic testing of the A/C system, specifically it's delivery or duct system, or duct leakage testing. This test, done with a Duct Blaster® or a DucTester, attempts to blow up a ducted a/c system like a balloon to check for, and quantify, leakage. In order for the system to build pressure, it's necessary to first seal off all the holes you know about - the supply vents - and then attach the fan to the return air side. The supply vents can be sealed with reusable air vent covers, like Vent Cap Systems, or disposable plastic tapes. The system is "inflated" to a standard pressure (typically 25 pascal) and the amount of air flow necessary to achieve that pressure is quantified and compared with standards.
"Envelope Tightness" refers to the space between the inside and the outside of a house, commonly referred to as "the building envelope." A large calibrated fan system, called a Blower Door, is placed in, and blocking, a doorway. The fan speed is increased until the pressure inside the house similarly reaches a given pressure, although in this instance it's typically -50 pascal, or the equivalent of a 20 mph wind blowing across your home. This allows you, with the aid of a smoke stick or other air movement detection, to actually locate and see leaks coming from unexpected places including in/around electrical outlets, plumbing penetrations, gaps in doorways, etc.. Although small, these leaks add up quickly and frequently equate to a giant hole in the wall.
Duct and Envelope Testing, and the new training/building requirements, attempt to limit this "leaking bucket" as much as possible. After all, if less air leaks out the home owner is going to be more comfortable, less air needs to be conditioned and the system isn't going to run as much resulting in lower monthly utility bills. Tighter systems also have less need for expensive, high efficiency equipment, potentially saving thousands in upfront construction costs. For frame of reference, the average house in the US leaks about 25%, of the air just conditioned, outside the building envelope. The savings from reducing that waste, the environmental aspect, and the reduced burden on overwhelmed utilities has led to bipartisan support for mandated building standards, education, and accountability - including DET.