well, singles don’t go in the album’s release group, they’re singles
All singles from an album would fall into that description, waterfall or not
The promotion status is used for releases that are not official, the examples given in the style guideline are pretty clear: a release with “not for sale” printed on the CD, included with a newspaper, or included with a magazine
There’s also some released specifically for DJs and some sent to radios - see relevant thread: Is radio airplay a release?
Also releases like this one released in advance of the official release for bandcamp subscribers (see edit #118543344)
And patreon-exclusive releases like this one