I'm writting a German specification (I'm not German).

Differences may appear for this process in different cultures, especially in the terminology, but usually here's the idea:

• The client writes his needs and wishes in a document, called a scope statement or requirements document.
• The supplier tries to understand the actual need of the client (which might be different to what was written and to what the client meant to say and to what the client thinks he needs, etc.)
• The supplier writes a specification for the product, which should fill the client's need.
• The specification needs to be precise enough for the product to be made (ambiguity problems occur).
• The client and the supplier can check whether they have understood each other, and discuss details of the product.
• The client agrees with the specification (or at least its current iteration) and the supplier is ready to start the work.

(it may of course be expected of you to disagree with this process, but this is irrelevant to my problem):

I'm now somewhere around the last two steps and I've been criticized because I wrote what the product must do, and not what it will do ideally.

Usually along the lines of

The product must be able to perform task A

And I was expected to write

The product performs task A

This is a simple word play, but I feel saying what the product does, while the product isn't even on the way to be made yet, is wrong. I would tend to consider a specification as a contract of what the product is expected to do (what it must do and how it should do it), and not what it does.

Said differently, I feel this is the specification and not the manual of the end product……

Should I say what the product must do or what it does?