Compared to software, the open source approach is relatively new to most actors in the field of (mechanical) hardware. Plus Open Source Hardware faces some special issues. A yet missing definition of its "source code" is one of them (+ patent law, liability, engineers that do not know how to work with git, costly prototyping…).
DIN SPEC 3105 will be/is the first official standard for Open Source Hardware and also the first official standard ever published under a free license (CC-BY-SA 4.0; that was a lot of lobby work ;) ). It defines the technology-specific "source" of Open Source Hardware and aims to build a bridge between research institutes, public authority, industry and the worldwide open source community.