I would say it better expresses doing something for somebody else(if the context doesn't imply doing something on behalf of...)

Hi, since you give a tech-engineering context, I just wanted to add that when a company works "per conto terzi" then this is often described as "contract work" in English; it's a common formula, with the myriad of Italian SMEs working as outsourcers for OEMs (often foreign!).
Cheers
Derek