JTAG is the underlying technology used in boundary-scan test (BST), that is used for manufacturing defect detection and diagnosis.
Normally, BST is used in benchtop environments, either as a standalone tester or integrated within a bed-of-nails or functional tester. However, when implemented in-situ on a printed circuit board, and controlled via a service processor-based JTAG controller, JTAG acquires much more power and flexibility. It can then be used throughout the lifecycle of a product, from prototype through environment stress screening (ESS), unencumbered by external hardware controllers, cables and fixturing. This results in improved yields, test capital cost reduction, test time compression, and enhanced defect diagnosability.