Changes to the original code, for example, have to pass a compatibility test.

ECONOMIST: Computer programming

It turned out that the inventor of the software had signed contracts with the company that allowed him to collect royalties even if the company made significant improvements to the original code.

FORBES: Magazine Article

The original game was just not designed to be expanded (no hooks in the original code) and so a full-game delivery mechanism was the only option.

FORBES: Connect