What is this

As the manual/handbook describes, the cards were used to test the Macintosh for correct function.

There was a Software that would be run on a Mac (II) that would run different tests with the various cables/boards attached.
The test software would then give some codes or even info on what part of the hardware was defective.

These are of little to no use nowadays. I have some complete Diagnostic kits that were officially given to mac repair centers by Apple.
The later Kits would have a standalone 'handheld' device that ran the tests without a second machine.

-Jonas