Written by Richard Lotte
Virtually every camera has this symbol Φ somewhere near its top dials. It indicates the position of the sensor (or film) plane. The distance between sensor and lens mount is called Flange Focal Distance (FFD from now on) and varies between camera systems. A Nikon DSLR i.e. has a pretty long FFD of 46,5mm. The FFD of a Mirrorless Nikon is only 16mm. The lenses of these camera systems are designed to work properly at their respective FFD. 
In general Mirrorless Cameras can do with a pretty short FFD. That's the reason there are so many adapters enabling the use of DSLR lenses with Mirrorless cameras. None the other way around.
FFD matters when you're using a view camera. The drawing shows a Cambo Actus with a mirrorless body attached. Indicated are the body's FFD and the distance between sensor and lens when focused at infinity; hence the ∞symbol.
Let's presume this lens is focused at infinity when it's 60mm away from the sensor. The camera - say it's a Sony A7 - has a FFD of 18mm. In that case the distance indicated as h1 is 42mm. That's nice. Camera movements - rise/fall, shift, tilt and swing - can be applied without risk of the lens' rear element touching the sensor.
The data sheets for Schneider and Rodenstock lenses generally mention the FFD at infinity and the length of the rear element. Add the camera body's FFD to the length of the rear element and subtract it from the lens' FFD and you've got the amount of free space behind the lens. Some space is needed to make the lens suitable for view camera use.
Traditionally the manufacturers of large format lenses also publish the size of the image circle. A lens wouldn't be of much use on a view camera if it would just cover the sensor. It needs to cover a larger area -the image circle - to enable camera movements, like shift. As long as the sensor remains within the image circle, it will produce a clean image without extreme vignetting.
Modern Schneider Digitar and Rodenstock Digaron lenses are suitable for use with digital backs. The longer ones will also work fine with Mirrorless camera bodies. Some of the wider lenses though have a FFD that's too short and/or a rear element that's too long to be used on a Mirrorless. Let alone a DSLR. The rear element would be inside of the camera at infinity focus. For your convenience we've published this lens compatibility chart.
Ѵ = OK - x = not suitable - Ѵ⁰ = focuses to infinity, limited movements - Ѵ1 = focuses to infinity, image circle too small to cover the full sensor
*GFX-100 mk1 requires a longer flange which compromises the lens choice. The table shows the lenses suitable for the current GFX bodies.