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.
In the past, the majority of Wincest fic have portrayed Dean mainly as the bottom and Sam as the top, and correspondingly Jensen as bottom and Jared as top in J2 RPF. Over the past few years, this appears to have changed, with Dean topping far more - possibly due to plot devices in which he is unusually aggressive, such as when he is a demon and when he's affected by the Mark of Cain. A 2016 poll taken by pathossam (a tumblr account with an admitted bias towards bottom!Sam) currently, the results released revealed that 121 people were in favor of Sam as bottom, as opposed to 103 in favor of Dean as bottom, and 88 in favor of switching roles or indifferent to them.
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 balance of the roles in both Destiel and Cockles fanfiction has remained fairly even since as to which partner is dominant. 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.
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: