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There are (at least were) two 'register.php' files, one is used by the 'profile' module. As barryC points out in
this post it's a good idea to add the code to both files.
All the code does is insist the request for that file comes from another page on the website, or a valid activation code is provided, which are the things that happen when a human registers.
I don't run any public sites myself, so it would be good to know if it makes the slightest bit of difference.
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