According to the Artscroll Selichot in the introduction, a שלמונית is "a piyut composed exclusively of quatrains, or four-lined stanzas."
This definition fits most of the selichot, maybe 75%, but most of them are not labeled שלמונית. In some cases this is because the selicha has another designation, like פזמון or פתיחה, which takes priority, but many times it has no label at all. The first example is selicha #2, אין מי יקרא בצדק.
Why are some of them labeled שלמונית and some aren't?