Guideline documents are documents that generally clarifies a directives or regulation, as Dr. Simon mentioned, they are not mandatory but they are considered and accepted as the best way to follow a directive. I would have to agree with most of the forum, that NB-MED guidance documents are the most important and useful to follow since they are written by Notified Bodies and they are the central piece of the EU regulatory structure and the ones that pass or fail the device for CE marking. These guideline documents are telling you what the Notified body will be looking for when inspecting a medical device.