Where an assembly of products exist, they may result in an overall new product, e.g. an assembly of machines either into a slightly larger machine or into a much larger process line. This assembly is most likely to require its own CE mark, name plate and DoC. In which case it must be clear on the DoC what the whole assembly is, especially if there are individual CE marks (and DoCs) of the smaller parts. Existing guidance suggests that photographs on the DoC can be used for clarity.
To be quite honest, apart from a few documents I’ve produced, I have not yet seen a DoC with a photograph on. It is said a picture is worth a thousand words for good reason.
For some types of products, a copy of the DoC should be included in the instructions. If so, a shortened form is acceptable, one that doesn’t include serial number information, however a full copy of the DoC should always be available on request.
If the DoC cannot achieve all the above, then it is either not the DoC for that product or it is insufficient.