Cite confidential source i'm currently using several construction drawings to extract a note on how a certain design is usually done in the industry. I'm neither allowed to name the specific product nor the company. I guess I can't assume that the extracted information are common knowledge, so a source or a footnote would be required. 

How do you typically handle this?
You can probably say something like: "A popular CAD* program treats the problem as follows...". Then use the footnote to explain that the program won't or can't be named for legal or other reasons. 

In other words, give a generic description but say that it is a specific example meeting that description. Don't give the impression that all do, unless you have evidence of it.