To specifically address your question, && is a Boolean And, while || is a Boolean Or. You can access very good inline help if you click the question mark to the right of the text "Ranking Formula" just before the formula entry box. That will bring up information on the Operators, Function Description, and Variables. Here is a partial screen capture of the information that is available.
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Yes @KaiBoon that is certainly another possibility. With contests really think about what your goal is, and how to structure a contest to meet it, both in what people do to participate and what people win. Also what is interesting to your community - put yourself in their shoes, or ask some regular visitors what keeps them coming back.
A contest could be around creating content that is of interest to others, who then share the content further, which then accelerates promotion of your community. What content do they create? Can it be something that already ties into the goals of the community, or some type of content that has resonated previously, or expands an area you've been meaning to move into?
And then there's rewards. This could be a special badge (have to be registered to get one of those 😉 or featuring the winner(s) or temporary special access to someone in the company or ?? Money or simple prizes are less powerful ways to garner participation. Essentially what we're talking about is the proper application of gamification - if done well, it accelerates motivation and participation. If done poorly, customers feel like they're being manipulated or coerced into doing something. Many more articles on this topic here, if you're interested.
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