In an anime/manga which isn't originally BL, the seme (top) and uke (bottom) are decided by the viewers. So there are no rules. There are seperate Victor X Yuuri and Yuuri X Victor fandoms. (I am fine either way, because I personally see Victor as someone who doesn't care about positions and simply loves Yuuri) There are also seperate Ereri and Riren fandoms (AOT) From Kyo Kara Maoh, it's WolfYuu and YuuRam. The more popular a fandom is, the more fanfics and Doujinshis it has. (The one who's name is written first is top and the other one is bottom) There are also reversible couples who take turns (like the main couple from Dark Heaven manhwa)
In an anime/manga which isn't originally BL, the seme (top) and uke (bottom) are decided by the viewers. So there are no rules. There are seperate Victor X Yuuri and Yuuri X Victor fandoms. (I am fine either way, because I personally see Victor as someone who doesn't care about positions and simply loves Yuuri) There are also seperate Ereri and Riren fandoms (AOT) From Kyo Kara Maoh, it's WolfYuu and YuuRam. The more popular a fandom is, the more fanfics and Doujinshis it has. (The one who's name is written first is top and the other one is bottom) There are also reversible couples who take turns (like the main couple from Dark Heaven manhwa)