Have you forgotten that you're the bottom in this relationship?
– Crowley, to Castiel, 6.22 The Man Who Knew Too Much
Slash fiction in most fandoms is heavily centered around defining the participants as either "tops" or "bottoms".
In fanfiction, the top has the penetrative role in a sexual act (usually both anal and oral), and adopts a dominant role in the relationship. The bottom is the receptive partner, and while not necessarily submissive in a BDSM sense, is usually portrayed as more emotionally vulnerable or fragile. While certainly not all slash fic complies to these roles, many do. Most fans include a Fic Warning for top/bottom configurations in sexually explicit fic.
The introduction of Castiel and the rise of Destiel as a pairing had Castiel usually written as topping when he was originally introduced as a powerful angel, then more often as the bottom when the series introduced his sexual inexperience in 5.03 Free to Be You and Me.
The roles remain balanced today, despite Misha Collins consistently advocating for Castiel in the exclusive role as top, such as in [an interview with blogger Carina MacKenzie in 2012] where Misha says, "Here's a spoiler for you, Carina. We do learn, definitively, in Purgatory, that Dean is a bottom." Or in 2014, at Vegascon, Misha [steals a fan's phone] and discovers that her phone background is a piece of Destiel fanart. He jokingly points to Dean and says, "That's me, the guy in the front," and has to be corrected by the fan that Castiel is, in fact, in the role of bottom in the picture. Also, Misha says in his last tweet that he likes being on top in reference to Cas/Dean fics reaching 100K on AO3.
Shipping fanfiction involving top and bottom roles may be found in all fic communities, but there are some communities dedicated specifically to a character exclusively in one role or the other, such as: