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.

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a lightweight, plain-text, markup language. It is used across the entire Stack Exchange network for post formatting (Questions, answers, wikis, chat) and comment formatting.
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For *either* meta questions about Stack Apps, or about how per-site metas are exposed by the API. (Stack Apps is its own meta site)
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For BUG REPORTS, not feature-requests. Use this when the API doesn't return data when the docs (or similar calls) say it should. This tag may be eliminated in the future.
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its own meta site. Use this tag to help differentiate moderation questions from the usual Stack Apps post.
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a server-side web application framework that uses JavaScript syntax.
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Questions about Stack Exchange notifications and the API routes that provide them. Not about apps that provide notifications - use [notifier] instead.
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Apps that provide notifications of things happening.
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an open standard for authorization.
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For questions relating to version 2 of OAuth, which is an open standard for authorization. See also the tag oauth.
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Apps, Scripts, Libraries, Bugs, Feature-requests, or APIs that are: deprecated, no longer available/supported, or no longer relevant.
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For questions related to the use of paging (returning a limited number of results, and using a page parameter to get the next batch) in the Stack Exchange API.
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a high-level, general-purpose, interpreted, dynamic programming language.
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Questions about using PHP code to interact with Stack Exchange and/or the Stack Exchange API. ... For general PHP questions, post on Stack Overflow instead.
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For apps that don't quite work yet, but need a registered Stack Apps post for testing. If you're following a tutorial which asks to post a placeholder app here, that's fine, but please delete it after…
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"Posts" covers both questions and answers. (And perhaps wiki pages) For example, you may use this tag for apps that help with general editing.
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Accompanies [app]s written in, or [library]s meant to be used from, the Python programming language
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Tools that help with questions, or issues with the API handling of questions.
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For questions about the maximum number of API calls per day, AKA 'quota'; 300 for unregistered apps, and 10000 for registered apps (per user when using personalized access tokens, per app otherwise).
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Questions about: throttling, the `backoff` flag, the quotas, or the rate limiting of requests made to the API. API calls are restricted both by per-second rates and by daily quotas.
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Questions about displaying things with minimal latency, or apps that do so.
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User reputation as returned by the API or displayed on Stack Exchange sites