confusing docs for externals

in:

http://svnbook.red-bean.com/svnbook/ch07s03.html

I find the following paragraph very difficult to understand:

  The support that exists for externals definitions in Subversion today
  can be a little misleading, though. The auxiliary working copies
  created via the externals definition support are still disconnected
  from the primary working copy. And Subversion still only truly
  operates on non-disjoint working copies. So, for example, if you want
  to commit changes that you've made in one or more of the auxiliary
  working copies, you must run svn commit explicitly on those working
  copies, not on the primary working copy.

Frankly, I can't make heads or tails of what this thing is saying.
Part of the problem may be that there is no definition earlier in the
document for "primary working copy" and "auxiliary working copy", but
I think it is deeper than that. Certainly some sort of examples would
help.

Could this paragraph be rephrased so that others don't have the same
problem in the future?

-- 
Perry E. Metzger		perry@piermont.com
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