Often used, more tracks than normal. So the disk-copy-software couldn't handle it. 
 
Detection
Software tries to read something from the disc. Could not read it. You can try it several times. Handling errors. It is no 1 or 0 it is an exception.
Copying
A copy makes a 0 out of the whole. And the software can detect this - it is nether 0 or 1. 
 
 
Often used - enter data from cover or description. 
With more and more photocopiers, often used bad copyable sujets (red etc) or complex systems like rotating discs (several lucas arts games  and were in the mood of the game).
Later the dialog to enter the number/code came in the middle of the game (manic mansion etc). 
Codes were also used to ‘store’ progression.
not used so often, expensive to produce. 
 
not known for 1980-1991.