pkg(8): difference between "-c /path" and "-r /path"

Hi,

This question is (as of yet) more of the "fill in background info" type:
From the "pkg(8)" man-page:

     -c <chroot path>, --chroot <chroot path>
    pkg will chroot in the <chroot path> environment.

     -r <root directory>, --rootdir <root directory>
    pkg will install all packages within the specified <root
    directory>.

up to now, I've leaned towards "pkg -c /path <cmd>" when
installing/updating a BE mounted at /path, but, given some results in the
past that weren't quite clear to me (yes, that's a bit vague, sorry), I've
been wondering whether "-r /path" would have been better.

I'd appreciate thoughts/advice/comments, TIA
Michael
-- 
Michael Schuster
http://recursiveramblings.wordpress.com/
recursion, n: see 'recursion'