The Machin Paper Varieties.

You can also add that the fluorescent particles have not been deliberately added, so therefore they are a contaminant within the structure of the paper and cannot be observed without the use of an ultraviolet light. WM

This exercise seems to have a lot to do with philatelic nomenclature. Philatelic nomenclature has previously been discussed in this community a number of times and I think the consensus is that the it is a mess. Different definitions abound, often tied to certain areas of the world and/or catalog publishers, and nomenclature can often change over time. I also think that many of us consider philatelic nomenclature through the lens of our own eyes; in other words, we tend to stick with the nomenclature that we originally learned and push back on changing what we 'already know'.

I also think that philatelic nomenclature is more important in technical threads than in non-technical threads. This makes sense, because in threads which are more detailed and complex it is important that everyone 'be on the same page' with the words and definitions that are being used. When everyone is not 'on the same nomenclature page', the result is that the thread often include many posts where the definitions are debated. There are many examples confusion in threads including terms such as; 'hue/shade', 'laid/ribbed paper', 'offset/setoff', etc.

One of the improvements that we could do is to present a short list of definitions in the first post made when starting a new technical thread.
Don