I'll throw this in as I remember reading it yesterday - straight from the Australian docs: "B757 and H47 (Chinook) are categorised Heavy (H) when the following aircraft is categorised either Medium (M) or Light (L) and categorised Medium (M) when the preceding aircraft is categorised Heavy (H)" (Jepp ATC AU-810 5.4.1.1).
Not that we have many, or any, B757s in Oz. Not sure if this is slightly different from other countries.