A very important part of product development is the realisation of an interim version, i.e. a working prototype. Unfortunately, the first prototype can only come out of the lab when the development process is almost complete, since most elements of the product need to come together in order for it to happen.
It is an important milestone, and very significant in terms of product assessment. Sometimes, as a result of the testing of the prototype, a reappraisal of the requirements (i.e. a specification change) comes about, which is why great attention should be paid before the project starts to thinking all the requirements through in detail. Specification changes are to be avoided if at all possible.
Once a prototype has been qualified, i.e. has met all the functional, testing, and certification requirements, it can be released for manufacturing.