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.

keywords or labels that categorize and group your question with other, similar questions.
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for questions related to determining what is in scope (on-topic) and out of scope (off-topic) for this site (GIS Stack Exchange).
For questions about, well, questions: how to ask, whether specific questions are appropriate, and so forth.
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for instructions and guidance on closing or migrating questions, and the reasons for those closures.
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Pertaining to the process of revising content or tags to bring better clarity to a question or answer.
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Use for questions related to using Comments against Questions and Answers
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The process of identifying and closing questions that are duplicates of another question
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for questions dealing with moderator actions, pages, abilities or decisions.
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held to choose moderators for this Stack Exchange site.
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for questions about any of the review queues
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a measurement of a user's contributions to the site, and the extent to which the community "trusts" that user with extra privileges.
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for questions regarding the design of the Stack Exchange websites; e.g., for issues with the layout. For questions about the user interface elements (inputs, checkboxes, etc), use the 'us…
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For questions about the user interface elements (inputs, checkboxes, etc) that make up this site. For questions about the design of elements, use the 'design' tag.
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for questions about the GIS Stack Exchange blog (to which any community members may contribute).
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allowable or desirable in question titles
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