At the last X Summit Fujifilm introduced the concept of “Value Angle” proposed to measure the quality of a mirrorless mount.

Never heard about before?

Without diving into too much engineering stuff: the greater the value angle, the more precise and easier a lens can send light to the sensor.
A wider value angle gives more flexibility for lens design and allows for more light and less digital correction.

In short: it expresses the potentiality of a camera system.

Fun facts: the way Fuji measures it the Sony E-mount does fine for APS-C while it ends up last for full-frame.
If you have a look at the table below, you’ll see Canon’s EOS M mount ranks first when it comes to this very parameter, i.e. it has the most flexible lens mount (please note: “Kleinbild” means “full-frame”).

Now keep in mind this isn’t an industry standard way to measure things. It’s “only” Fuji’s way to measure a mount.