That is about a public tender, where there are "form requirements", meaning that certain documents and signatures have to conform to the required form. Usually that means "in writing, on paper, with signatures made by hand" or the new legally accepted replacement of secure electronic signatures. Sometimes it might even mean signing the paper in front of a notary, e.g. for contracts regarding the sale of real estate. But in general, outside clearly defined exceptions, contracts and declarations do not have a form requirement, so even oral agreement or "conclusive behaviour" (sign language, point-and-grunt, silently carrying an item to the cash register) are absolutely valid.

Of course, when in doubt, proving the exact content of such an unwritten contract may be hard.