unity-system-compositor (USC) is versioned using a standard x.y.z versioning scheme, where x is the major version, y is the minor version and z is the patch version.
The minor version should be increased at least every time USC is changed to use a newer mir server or mir client API/ABI. In other words, we want to guarantee that all patch releases x.y.* will continue to work with the same mir library ABIs that x.y.0 was originally released against. This scheme makes it straightforward to manage updates/fixes for released versions as patch releases.