I noticed that in the profile of people with high reputation, they have a percentage of the overall Reputation at the right of the Reputation section.
What does someone need to get that information in his profile?
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You need to be ranked in a league; to be ranked you need at least 200 reputation.
Not all sites show the link in to their reputation leagues in user profiles; I think there is an option for moderators to opt out for their site, or it is simply not shown for sites with too little 'movement' in reputation to make for an interesting league table just yet.