Parameter used (mainly) in CMOS based consumer cameras.
Cameras until about 2010 - Gain and ISO is the same thing
Newer cameras - example - Nikon D850 - only 3 gain values - ISO 64 - first gain value - best DR → 14-bit - ISO 100 - second gain value - ISO 200 - second gain value + multiplies pixel values by 2 - ISO 400 - third gain value - higher ISO values still use third gain value
ISO is therefore a combination of hardware gain and digital value multiplication
ISO independent - single gain value, everything else only pixel value multiplication - examples - Nikon Z8, Z9 - Sony Alpha 9 iii