That means it was a closed, proprietary system that was reverse engineered & emulated. Different from an open system that doesn't require that. Also more efficient in hardware if any clones port original design.
C64: http://www.classiccmp.org/cini/pdf/Commodore/C64%20Programme...
C128: http://www.pagetable.com/docs/Commodore%20128%20Programmer's...
Because that's still not equal to an open-source design where the licensing eliminates such risks.