A tag is a keyword or label that categorizes your question with other, similar questions. Using the right tags makes it easier for others to find and answer your question.

Type to find tags:
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designed to solicit opinions or best-practices on a particular topic, with the goal of reaching community consensus.
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You need help with the use of one or more of the site's features.
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You have a question, concern, or suggestion about a tag or how tags are used on our site.
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You have an idea for a new feature, or for a change to the existing functionality.
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Indicates you've found an erroneous or unexpected behaviour in the system that needs to be fixed.
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Indicates that a feature request has been implemented, a bug has been fixed, or another type of request has been processed.
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actually due to the existing design of the system and is not considered erroneous behaviour.
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Questions about tag synonyms (how they work, who can create them, synonym suggestions etc).
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Questions related to policies and guidelines of the Stack Exchange sites.
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when examining what is on-topic or off-topic, and why.
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Questions about answers: how to answer, whether specific answers are appropriate, and so forth.
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for general questions about questions that have been closed because they are off-topic or otherwise do not meet the criteria defined in the FAQ.
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small addendum to each question or answer, intentionally limited in size and formatting.
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"Design" refers to front-end elements, such as page layout, coloring conventions, etc.
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For questions regarding the nature of a "question" on Cross Validated; e.g. how to ask, whether specific questions are appropriate, etc.
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the process of moving threads from one Stack Exchange site to another.
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Indicates that a request (usually a feature request) has been considered, but will not be implemented. Generally speaking, a developer will leave a comment as to why, or the community will come to con…
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Questions dealing with moderator actions, pages, abilities or decisions.
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For questions about the process of editing, how edits work, and other general inquiries about the edit system, used in editing questions, answers, tag wikis...
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The process of identifying and closing questions that are exact duplicates of another question
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a flag that can be applied to answers (and sometimes questions) which lowers the barrier for editing and prevents any rep gain/loss from voting.
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Cross-posting refers to posting the same question on multiple websites.