Mass customisation with 3D printing is production in which every item is different from the next, yet the items are made in very large numbers. The best-known example is clear aligners, the transparent plastic trays that move a person's teeth into place. Each set is made to fit one patient, and the maker produces them by the million.
Mass production makes millions of identical items, for example with injection moulding. Its mould is expensive, so it pays only if the same part is made in great numbers. A 3D printer needs no mould. It can print a different shape every time, from a new file. So a factory can print thousands of different shapes a day, with no extra tooling cost.
The process needs software at every step. The dentist scans the patient's teeth, and the scan becomes a 3D model, as with clear aligners and tooth scanners. Software plans how each tooth should move, in small stages. For each stage, a mould of the teeth is printed. A sheet of plastic is pressed over it and trimmed to make that stage's aligner. The trays are labelled, packed and sent to the dentist. Dental crowns, which are milled and printed to order, follow the same idea.
The same idea applies to other things made to fit the body. Imagine a maker of shoe insoles in Nagpur for people with diabetes, whose feet need an exact fit. A clinic scans each patient's feet. Software shapes an insole to match, and a printer prints that pair in a few hours. The next patient's pair has a different shape, and it costs the maker no new tooling.
This is a very large use of 3D printing, and it depends on automation. Nobody can plan millions of unique items by hand. The model of each item is drawn from the scan by software similar to CAD, with little or no human work for each item.