IO and CPU bound are meant in the sense of upper and lower bound, not destination, but limits on the system.

Saying something is CPU bound means that the CPU is the bottleneck. For instance, the data IO doesn’t limit the system throughout of iteratively running a hash algorithm a million times, the CPU does. An IO bound process might be something like reading a lot of data off a disk (or over a network) with a small, quick transformation on the CPU. Where you saturate the data bus, but the CPU is comparatively idle.