The point here is to check that when AMC reads 1/1 (page 1 from copy number 1) from the binary boxes at the top of the page, this is really the page 1/1 and not some random data that could have been read after an incorrect detection of the 4 corner circles. So if AMC reads 1/1 but not 60 for the control code, the scan is considered as unrecognized.

Why does it start at 60 and count down for each copy number of the questionnaire.

The control code should be different for different pages, that's why it is decremented. 60: why not? You can change this value if you think another one should be better.

What happens when it reaches zero?

It goes to 60 again.