So I've been tagging a lot and was wondering, what are the rules, if any, about making "new tags?"

Just for an example:
Say, if there were a pic of a fur standing on a roof, could one make a tag like standing_on_roof?

A tag is commonly worthwhile if it is specific to the image and sometimes worth searching for. If not, and if there's so few of them (let's say 10), then there's no real point to the tag.

..Unless it's important, such as identifying the name of a character, or a particularly odd species, such as the dorukolorukalai.

PapaFranku said:
could one make a tag like standing_on_roof?

Try using existing tags like standing & roof/rooftop.

If you think of a complex tag try breaking it down to smaller already existing tags, i could also start listing a bunch of tags that i would never imagine that such word existed before actively uploading and tagging here.

But one my favorite tags that i had never thought to exist is puffy_anus

In any event of you asking yourself if such tag exists or not i'd suggest these two things:

One you could actively search for a tag in the tag list or the wiki at https://e621.net/tag and https://e621.net/wiki/show?title=help%3Ahome respectively.

The second one i use is a script made by a user here of e621 named Xch3l that can be found at forum #66904 which has one feature which allows you to search the existence of a tag while tagging! (It's pretty handy!)

I hope i could be of help to you!

You could technically. But there's a couple of things I'd recommend doing first:

• Check the tag tool with a wildcard search to make sure that there isn't a similar tag already in use. Examples: stand*, *roof*
• Think about how helpful the tag might be to someone. For example, are they more likely to search roof/roof_top + standing instead
• Try not to be too specific. Remember that characters can stand on a potentially infinite number of surfaces and some may be more common than others. Sitting in a chair or a car would be something you can find a lot of pretty easily, but sitting on a davenport would probably be pretty uncommon and who's gonna search for that specifically?
• Ask the forums if you think you have a good idea for a tag. It's possible that there might be a tag that already exists that does the same thing, or others might be interested in helping you tag them.

Just_Another_Dragon said:
If you think of a complex tag try breaking it down to smaller already existing tags,

Please don't do this