Testing, Adjusting, and Balancing (TAB) is the commissioning step that measures and adjusts an HVAC system's air and water flows so every zone actually delivers the airflow the engineer designed. Without it, a building can have brand-new equipment and still run with hot and cold spots, short-cycling, and rooms that never reach setpoint — the equipment is fine, but the distribution was never proven.
The AABC (Associated Air Balance Council), founded in 1965, is a TAB certifying body whose defining feature is independence: its members are strictly prohibited from any affiliation with mechanical contractors, design engineers, or equipment manufacturers. That matters because a balancer affiliated with the company that installed the system has an incentive to report that the install passed. An independent AABC firm is a third party with nothing to defend, which is why owners and engineers specify independent TAB on serious commercial projects. AABC certifies the Test & Balance Engineer (TBE) and Certified Test & Balance Technician (TBT).
Other recognized TAB bodies exist — NEBB and TABB — and they certify competent firms; the distinction is that they permit certified firms to be affiliated with mechanical contractors, while AABC does not. For an owner commissioning a new system or chasing a comfort complaint, a balance report from a certified, independent TAB firm is the document that turns "we installed it" into "we proved it works as designed."