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prefered domain thing is mostly for sorting out the duplicate content issue.
for example, your site might respond to www.abc.com/1.html and abc.com/1.html and xcv.abc.com/1.html with the same file.
google thinks you're duplicating the same content on 3 subdomains.
so by setting that you tell google which one is the correct copy which one is the duplicate to ignore.
if I were you I'd check my backlinks and currently indexed pages at google and choose the subdomain that is most indexed and backlinked.
this way you wouldn't lose your PR and indexed pages for the ones that you didn't choose.
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webgeek
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