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Does not need to be the first tag a.k.a. category. And if you don't use the tag with palnet.io you can only add four tags to your article or the frontend will tell you, you've used more then five.

If post is created on Steemit, use tag palnet.
If post is created on palnet, the tag is automatically added.

Yes, and it's not a problem to add it any way. If you post via palnet.io and you put in four tags plus the tag palnet it is just fine and will go through.

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