Waterfall currently has three special case tags that modify the behaviour of posts. These are the "Do Not Reblog" tag, the "Do Not Interact" tag, and the NSFW tag.
Special case tags are case insensitive.
Do Not Reblog
Do Not Reblog (DNR) is a tag that can be added to posts to prevent them from being reblogged. There's no set criteria for when a post should be marked as DNR, and is at the poster's discretion.
When this tag is added to a post, the site actively prevents it from being reblogged. Users can still like or comment on the post, however.
Adding the DNR tag after a post has already been reblogged will not remove the reblogs, and does not prevent the post being reblogged from reblogs already made unless they too have the DNR tags set.
Both the full phrase - "do not reblog" - and the acronym "dnr" are considered valid tags that will trigger DNR mode.
Do Not Interact
Do Not Interact (DNI) is a stricter version of the DNR tag, which also prevents commenting on the post. Users can still like the post, however. This is intended behaviour, as a common use for DNI posts is talking about personal issues and the use of likes allows a relatively unintrusive way of expressing condolences.
As with DNR posts, both the full phrase - "do not interact" - and the acronym - "dni" - trigger this mode.
This tag is mandatory on posts that meet the criteria listed here.
The NSFW tag marks a post as containing adult content, and prevents it from being shown to minors, or adult users who have opted out of viewing adult content.
The prescribed tag is "nsfw" - variations are acceptable when used in addition to this base tag.