Crowley: (to Castiel) Have you forgotten that you're the bottom in this relationship?
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, the majority do. In Wincest, overwhelmingly Dean is portrayed as the bottom, and Sam as the top. Correspondingly it is usually Jensen who is portrayed as the bottom and Jared as the top in RPF. Roles are generally less rigid with other pairings.
The introduction of Castiel and the rise of Dean/Castiel as a pairing, did disrupt these designations. A large number of Dean/Castiel authors had not written, or indeed read, Wincest and so were not as restricted by the well-established tropes associated with that pairing. Castiel is usually written as topping when he’s being powerful angel, or as a bottom when the fic focuses on his sexual inexperience. There is typically more variety in the roles in Destiel fic.
In an interview with blogger Carina MacKenzie in 2012, Misha said ""Here's a spoiler for you, Carina. We do learn, definitively, in Purgatory, that Dean is a bottom." source.
Some fans will include a Fic Warning for stories containing the "aberrant" Top!Dean or Bottom!Sammy.
These fics will be found in all fic communties, but there are some specialist communities: