Everyone is a possible expert, but the leader board shows the top "earners" of the title expert.
Quote from the link you linked:
The leader board recognises the top experts for the current week, and the top experts of all time. There's also a leader board for each topic, to recognise experts in specific fields.
As a Jira power user, I was at first doubtful that Trello could benefit my workflow. Jira already uses boards (ones you can customize!), so why would I even need to use Trello?! In this post you will...
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