Cogset Interface

The cogset interface determines how a cassette, freewheel or sprocket can be installed onto the rear hub. There are a handful of interfaces but very few are cross compatible.
In order to secure a cassette or sprocket onto the freehub body it requires a lockring that threads into the end of the freehub body.
The orientation of each individual sprockets in a cassette affects shifting performance so most cassettes and sprockets can only be installed one way.

For example:
An 11spd Shimano Road cogset interface has 9 notches. 1 of those 9 notches is smaller than the others so the small notch needs to be aligned on both the cassette and the freehub body.