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Irving's correct - that tag is basically nonsense, as best I can tell. It's just a comment. There's no approved tag to block part of a page from Google. Google has a directive that looks similar to this, but it's for Google Search Appliance (internal site search) and doesn't impact organic search (we actually just tested it recently, coincidentally). I disagree a little bit about cross-domain canonical. I have some friends at major media properties who have had good luck using cross-domain canonical for syndicated content, and it does seem to pass link-juice when used appropriately. It is true that Google doesn't always honor the cross-domain use, so it's very situational. 301-redirects are certainly more reliable, but they are different in the sense that a canonical will only remove the source page from search, whereas a 301 will remove it for users as well. So, it really depends on what you're trying to accomplish.