As time goes by, we sometimes find it's good to have rules written down for things relating to how we operate. This is where we write these rules. Please don't change them without prior discussion.
It's pretty simple: if a plugin is, or will shortly be (eg. author giving heads-up about imminent release), publicly available for download, it is eligible for addition to the appropriate masterlist.
Plugins that are not available to the public are not suitable for inclusion in the masterlist - if someone reports such a plugin, tell them that it is more appropriate for them to add any necessary metadata using a userlist rule. Such cases include private beta plugins, or more generally plugins distributed amongst a select group of people, or plugins for which you have to pay to access (which is against Bethesda's rules, but has undoubtedly happened before).
Sometimes it is necessary for a maintainer to download a plugin to investigate it, eg. to double-check information provided by a user. In such cases, we generally ask the user to provide a link to the plugin in question. However, because most submitting activity occurs on Bethesda's official forums, we have to abide by their rules. These rules include not linking to nude mods, for instance, and not discussing certain mods that break their rules (eg. Morroblivion).
If a user submits a plugin that falls into such a catagory and they provide a link, ask them to remove it, stating why. It's best not to draw negative moderator attention to the threads, because we need them to stay open. (Moderators are highly unlikely to lock the thread or get on to us about such cases, but better safe than sorry, plus you educate users that may have been unaware of the rules.)
If a user submits a plugin that falls into such a catagory and they don't provide a link, ask them to PM you a link to the plugin.