Re: plain/simple/easy language variant subtag

How is "language level proficiency" determined? 
I see how CERM is described (See Note 1), but how is it measured?.

". . . One variant on your proposal is that I would suggest you make the 
semantics to specify a "maximum required level" where possible - e.g. 
someone with a "B1" proficiency in German could expect to read something 
in "de-cefrb1" without a lot of difficulty...
This is not as simple as trying to have "en-plain". But it is probably 
easier than trying to agree on what that would mean,

How would EN-B1 different than EN-plain? 

If (and that is a big if) language levels can be objectively determined, 
then the "maximum required level" could be used to tag the page, content 
or parts of content. 

Note 1: 
http://www.deutsch-als-fremdsprache.org/en/faq/323-what-does-language-level-a1-a2-b1-b2-c1-and-c2-mean.html

CERF: The Common European Framework of Reference for Languages (CEF or 
CEFR) was put together by the Council of Europe as a way of standardising 
the levels of language exams in different regions. It is very widely used 
internationally and all important exams are mapped to the CEFR. There are 
six levels: A1, A2, B1, B2, C1, C2. These are described in the table 
below. . . 
See http://www.examenglish.com/CEFR/cefr.php   
____________________________________________
Regards,
Phill Jenkins, 
IBM Accessibility