@huige I do not think you can tag posts for now... just topics... please check some relevant discussions on the topic below...
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Tags for posts in addition to topics
Over at mangolass.it (managed by @scottalanmiller) we have long had problems with search just completely sucking. Has anyone ever looked into the feasibility...
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NodeBB Community (community.nodebb.org)
hopefully in the near future though 😄