Good question. It turns out that digital sensors (and focus) has much tighter tolerances than the film plane. Much less. It takes a while to get one's head around this.... The problem was uncovered c. 2005-2010 when all sorts of efforts to have flexibility (such as Leica DMR, with interchange between digital and film) were found to be difficult to hold tight, and thus Hassy and Phase One went more to closed systems (their own backs on their own cameras) to maintain tolerances. Alpa uses found the need for micro-shims to adjust their backs to their HPF focal rings to get that extra bit of sharpness (there's an essay by Mark Dubovoy on this on LL, and Michael Reichman wrote on it here, under s
himming:
https://luminous-landscape.com/alpa-stc-review/).
Rollei's answer in the Hy6 was to implement adjustment of the focus plane as each lens (surprisingly) varies. This has been adopted by other manufacturers, as it can be found deep in the sub-menus of Canons, etc. Its largely to correct the AF setting for those lenses, but can also be used to set the focus confirmation for manual lenses. Its like tweaking your camera focus for each lens - although you have to spend the time testing the camera and lens with the digital back to get the proper setting. Just to confuse things, some lenses change their focus posture (called focus shift) as they go from near to far (the 300APO is the big offender here, as are early Leica Summilux), as the offset setting for near and far is different - but in general, most are just fine once they are set up.