Now it's "film photography", the province of hobbyists, some die-hard holdouts in the art photography world, and also hipsters who have injected a lot of hand-wavery and woo into what was just a fairly straightforward and scientifically understood process. Kind of like tube amps used to be just "amps" and now there's... but that's a whole different thing, and one on which I'm not at all qualified to comment.
I am, however, qualified to talk about how minilabs worked, back in the days when they were still wet-printing with light shone through a negative onto photo paper.
Basically, you'd have different "channels" on your printer for different film types. To calibrate the channels, you'd shoot prints of these test negatives (called "Noras" for reasons that should be obvious) and then check the grayscale target with your densitometer, which would let you know if the calibration had drifted.
You needed the different channels because films handle color differently, and even have different tinges to their substrates. (Normie Fuji negative film, for example, has a more magenta tint to the negative as opposed to the familiar orange of normie Kodak Gold.)
You'd then twiddle things into calibration and be off to the races.
As long as your processor was running right and your channels were properly calibrated, gray would be gray, no matter what film you put through it.
That was the science part of it.
The art part was that lab tech sitting there squinting into the glowing window and looking at the negative and... if they were good and they cared ...making on-the-fly corrections as needed. If you have old prints from that era and look on the back of the photo, you might see a string of dot-matrix printed digits something like "N +1 N N", which indicate what adjustments had been made from the default channel setting.
I considered myself pretty good, and the paper waste numbers on my shifts backed me up. See, you'd weigh the contents of the shredder at the end of a shift. Having to reprint a whole 36 exposure roll of 4x6's put a fair amount of paper in there, and that Kodak paper wasn't free.
It was harder to notice in normie color photos unless there was a lot of sky, or people wearing white, or whatever. One place it was hard to hide, though, was if you'd had some of the C41 process monochrome film printed: Kodak BW400CN or Ilford XP2. This was black and white film that was designed to be processed in color chemistry. It's hard to hide your colors being out of calibration on your channel when there shouldn't be any color in there in the first place.
.
