I'm not having a good day today!
Like an *****, I posted a reply, but I did it on the post that was linked previously!
Anyway, here it is:
Softwood is graded 1 to V1, effectively for appearance (nothing to do with structural strength).
Grade 1V is ok in larger sections, where a few minor defects can be tolerated e.g. doors etc.
V and V1 are as pointed out above, pretty much firewood, but are ok for shelves in the shed, hen houses, bird boxes etc.
These 3 are effectively "sorted" as they are pulled out of the timber stack when it is delivered to the yard or when softwood is being sorted for delivery to the customer.
Grades 1, 11 and 111 are all lumped altogether as "unsorted", as they are what is left once the inferior grades have been "sorted" out. If you want the very best from amongst these you probably need to ask if you can select it for yourself and of course, pay a lot more!
CLS is "Canadian Lumber Stock" which is from quick grown pine and is suitable for carcassing etc. From my experience it seems to be relatively knot free, but still isn't very good looking and for or some reason which beats me, it generally seems to come with the corners rounded off!
For constructional work, softwood is "stress graded" to various British Standards and that is what things like "G2" and "C16" are all about.