Assuming that “Expression de besoin – document de travail” is the title of a document, it would be better to use “expression de besoins” or “expression des besoins” which are literal translations of “statement of requirements” and is a step (or the product of a step) in some IT project management methods.

What I can think of is “l’expression d’un besoin”, that is, someone is in need of something (besoin), and expresses it. The result, what he said or wrote, then is “l’expression d’un besoin”.

That said, I think we lack some context, it could wel lbe something else I didn’t think of.

In a software development context, this document is part of the requirements and is the starting point before writing code.

The main client (the owner of the application) generally writes an “Expression de Besoin”
then the “Maîtrise d’ouvrage” will translate the needs into functional specifications and finally the IT contractor (“Maîtrise d’œuvre”) will write some technical specifications then code what is needed to answer the needs.

In real life the IT contractor generally goes straight to code without writing technical specifications.

I think this should be pretty close to the SRS, which is a standard document from IEEE that defines the framework of the software development through discussion between the customer and the contractor. Since the customer is king, in French, he is expressing his needs in this “expression de besoin” document that the contractor will have to fullfill.

J’utilise le document “l’expression de besoin” pour transmettre mes commandes vers le fournisseur. C’est un document officiel qu’on signe avec le fournisseur.