Conformance levels and core /extended

I do not see how the "impossible/difficult/easier approach worked out in WCAG 1.0" corresponding to P1, P2, P3 terms is different from core and extended. Core checkpoints signify that they are vital to accessibilitty  just as the P1 or "impossible to access" checkpoints indicated in 1.0. 
  What is termed as extended now are supposed to enhance accessibility/usability just as P2 and P3 checkpoints did.
  I fail to see how they are "very different"  from the P1, p2 scheme and why the  WCAG1 scheme is "untenable". If   "conceptions of conformance levels have changed so radically " since 1.0, these are not brought out in the WCAG 2.0 requirements doc nor explained in the WCAG 2. I believe it is really important to convey these differences especially to advocate transition from WCAG 1 to 2.0. Can I turn to some document for the new concepts underlying WCAG 2? Nor have the terms core and extended been defined anywhere in WCAG 2.0. The choice of terms "core" and "extended" certainly need further  deliberation and justification.
  Sailesh Panchang

  On Tuesday, September 16, 2003 12:07 AM Jason White  wrote 
  Subject: Re: Conformance levels and best practices
  "Others have argued, however, that precisely because the conceptions of conformance levels have changed so radically between WCAG 1.0 and WCAG
  2.0, the same terms (Priority 1, Priority 2 and Priority 3) should not be
  carried forward into the 2.0 document, as this would create confusion for those who are already acquainted with the 1.0 conformance structure and its definitions.

  In developing WCAG 2.0, the distinctions underpinning the 1.0 priority definitions were subjected to detailed, critical scrutiny, with the result
  that the working group decided they were no longer tenable for purposes of
  the 2.0 document, and that a new set of distinctions was needed to classify checkpoints and success criteria in constructing the 2.0
  conformance scheme. The argument for new terminology, then, is that unless
  different terms were used, readers acquainted with 1.0 would construe 2.0
  as following the same, or a similar, system of priorities and conformance levels as WCAG 1.0, and would make policy decisions and claims about
  accessibility on that erroneous, but tempting, presupposition. The new
  terminology is supposed to signal the fact that the 2.0 categories of core and extended are very different from the 1.0 priorities, and that the
  impossible/difficult/easier approach worked out in the 1.0 conformance
  scheme does not apply to 2.0."

  My purpose in stating these arguments is not to defend them, though I do
  happen to find them persuasive, but to attempt to recapitulate the reasons
  which led the working group to the conclusion that new terminology was
  needed and that retention of the old terminology would do more harm than
  good.

  Comments, counter-arguments and proposals are, of course, most welcome. If
  anyone is dissatisfied with the working group's prior decisions on this
  matter, the issue should be re-opened in the context of our ongoing
  conformance discussion.
  "





  On Mon, 15 Sep 2003, Sailesh Panchang wrote:
  >
  > 1. I believe the words Priority 1, 2 and 3 with their definitions conveyed  that the focus of the checkpoints was on successively increasing levels of   accessibility ... vital -essential-desirable for accessibility, in a way better than that conveyed by the  terms coreand extended. Admittedly, there   are different perspectives on the priority levels assigned to some checkpoints in WCAG1. But a P3 checkpoint  enhances accessibility and therefore usability and shows the way ahead as is intended for "best practices".  So why not  stick with the terms P1, P2 and P3... it will also be   "backward compatible"... Goal 6 for WCAG2 in requirements doc. The required success criteria should be  categorized as  P1 or P2 or P3.