For questions about the required tags on meta sites and Stack Apps: on most sites, they are [discussion], [support], [bug], and [feature-request].
Each new question posted on Meta Stack Exchange and most per-site metas requires at least one of the following tags:
Area 51 Discussions and Stack Apps also have required tags, but they are different from the ones above: on Area 51 Discussions, required tags are ones for all of the different "subject areas" listed plus [area51-meta], and on Stack Apps required tags are used to differentiate between questions that actually ask questions and those that simply publish programs, and for subcategorization within both of these.
Use this tag here for questions about how required tags work on all sites, or for general questions about the four required tags used here and on per-site metas. For specific questions about their use on a specific site, or about the different set of required tags on Area 51 Discussions and Stack Apps, ask on the respective sites themselves, using the appropriate tag to indicate that it's a meta-meta question.