A customer in São Paulo orders a jacket from your shop. Your checkout asks for a name, an address and a card, and the order goes through. Two weeks later the parcel is still sitting at Brazilian customs, because since 2020 Brazil's postal service has required every international parcel to carry the buyer's CPF, their individual tax number. Your form never asked for it. The customer has to find a government import portal and type it in there, or the jacket never arrives.
Now turn it around. A tourist from Germany tries to buy a train ticket from a Brazilian website. The CPF field is required, they don't have one, and that's the end of the purchase.
Names and addresses get most of the attention, and the last two chapters covered them. But forms ask for plenty more: ID and tax numbers, titles, gender, dates of birth, consent, and proof that you're human. Each of those has a version that only makes sense in one country.