Previous recommendations as to how and what tag by Hydrus Developer are still recommended to give a read since they're a great baseline to start from.
Still, after the PTR move, the tag repo is now handled by a group of janitors in order to make the sorting of petitions easier and follow agreed guidelines.
As such, the next namespaces are the ones that are the standard


 Character: The name of the character appearing in the content you're tagging. 
            When possible, do try to avoid tagging a character with their series in the name. This is obviously impossible for characters from the fate: series for example.
            Related to point above, characters can have an official subtitle that will help differentiate and create siblings for them. So for example, Akali (lol) should be tagged with Akali, the fist of shadow in order to avoid confusion.
 Creator:   It is used for content in which there's an defined artist. 
            For example, creator:dandonfuga or creator:yeero. Despite one being 2D drawings and the other 3D renderings, they're still the creator of this piece. They created this one using others materials.
            The material used can be copyrighted, but this is handled through the use of character, series and studio.
            This tag is NOT used for IRL content. Please refer to person/idol below.
 Studio:    Studio that created the content.
            For example, for a character from the Final Fantasy series, this will be Square Enix.
            For IRL, this refers to the studio that handled the creation. So for example, studio:graphis
 Medium:    This is related to the medium or details of it in which art is created.
            For example, medium:real life for images that are from RL.
 Meta:      Basically metadata for the image. 
            This includes stuff like meta:patreon reward since it's not the medium in which the art is created but still gives information as for the source and quality of it.
 Species:   This tag is exclusive for the species of the character/s appearing in the content.
 Person:    Tag applied when the image has a real person, either pictured in real life or in a 2D drawing. If the person has an "alias", add it to the tag itself.
            For example, Kato, a steampunk model takes a selfie. That would be person:kate "kato" lambert. If you drew her, it would still be person:kate "kato" lambert. Do use medium: or other unnamespaced tags in order to differentiate them.
            Currently, there's a split on idol/person but both are accepted and correct. Do feel free to hide one through tag viewer preferences.
 Photoset:  This namespace is reserved for IRL content in which all the tagged images with the same tag belong to a single photoset.
            For example, photoset:darkest night from person:kate "kato" lambert

The following tags are REMOVED ON SIGHT and are not accepted in their current form. Do feel free to bring them up in a discussion with janitors and other users in the Hydrus Discord.

Namespaced tags: ANY NON OBJECTIVE TAG / TAG THAT IS LOCAL ONLY,  Action, emotion, class, outfit, head, face, hair, arms, breasts, legs, feet, navel, nipples, girlscount, malescount, boys, girls, group, circle, artist, parody, copyright, franchise, eyes, urine, uploader, filename
Other tags that are frowned upon and usually removed: banned artist and their variation [...]

Related to tag petitions, any petition in which their reasoning is not a canned one (like mangled parse/typo or should be namespaced) and actually a written one, has a much higher chances of being accepted. Janitors cannot see all files, so we obviously do not accept petitions that want to remove the only creator: tag from files we cannot see without reason for example.


Finally, a small reminder that siblings are a system implemented in Hydrus and if you're petitioning a tag that could be solved through a sibling like natsuverse:fate stay night -> series:fate, that will help out the community as a whole.