Generally siddurim for European Jewry are labeled nusach Ashkenaz or nusach Sefard. However, Selichos are often labeled nusach Lita or nusach Polin. My understanding is that Lita is Ashkenaz, and Polin is Sefard. In other contexts, it seems that nusach Ashkenaz might be broken down into Lita and Polin sub-dialects. Even inside a standard Sefard siddur there are two versions of the Kel erech apayim prayer when taking out a sefer Torah, titled Ashkenaz Lita and Polin versions.
I'm a bit confused how this taxonomy works. Can someone explain it to me please?