Apple Service Diagnostic (ASD) is supplied by Apple to Apple-authorized service facilities.
It is a verification diagnostic with multiple test options, such as self-running and user-interactive testing. It can perform tests without the need for an OS and can also test functions that incorporate the OS.
Each version of ASD is designed to work on a limited number of Mac models and is downloadable from GSX.
ASD can take several hours to finish, depending on the options chosen, but generally does more extensive testing than components of Apple Service Toolkit.
It is generally recommended that ASD be run for verification of repairs.