Basically that software can be patented.

Back during the 70s-80s, a specific set of code was covered by copyright. But the behavior encoded could be reverse engineered.

A famous example is how Compaq clean room reverse engineered the BIOS of the IBM PC, thus opening up the market for the clones.

In constrast there are all kinds of issues with using FAT (FAT32/ExFAT) variants on Android phones because MS have/had patents. And i think Android early on didn't offer things like pinch to zoom because Apple held a patent (not sure what happened there).