What is "Scan"?
Scan Insertion and ATPG helps test ASICs (e.g. chips) during manufacture. If you know what JTAG boundary scan is, then Scan is the similar idea except that it is done inside the chip instead of on the whole board. Scan tests for defects in the chip's circuitry after it is manufactured (e.g. Scan does not help you test whether your Design functions as intended). ASIC designers usually execute the scan themselves and happens just after synthesis. ATPG (Automated Test Pattern Generation) refers to the creation of "Test Vectors" that the Scan circuitry enables to be introduced into the chip.