"snuff" means that one of the characters in the story is killed or commits suicide, and that this act is more or less the main focus of the story. So for instance Game of Thrones is not snuff, despite all the death, whereas many stories in /lit/ are.

As far as I know (non)con is used for both sex and death.

The "scat" tag probably wouldn't be warranted. Think of it like this: tags are used so that fans of a certain genre can easily find stories that appeal to their interests. A fan of scat would probably be fairly disappointed after finding they've read a few thousand words to only get a half sentence about the topic they were expecting.

Most stories have about two to six tags, so that's probably a decent range to aim for. Make sure to include keywords that are the main focus of your story. So if, for example, half of your story deals with the process of somebody getting their limbs cut off, you'll want to include a tag like "amp" or "amputation". If something is a relatively small part of the story, there's probably no need to include the tag.

Generally useful tags are the genders of the participants, e.g. "f/f" to indicate a story with mostly lesbian content, the amount of consent (con / semi-con / non-con) and the "loli" or "shota" tag if you have underage girls or boys, respectively. The snuff tag is optional, since someone dying is more or less the default on here (which is a shame, if you ask me).

Aside from that, your best source of ideas for tags are probably to look at the tags that other stories use.