212.4d Some enchantments have the subtype "Aura." An Aura comes into play attached to a permanent or player. What an Aura can be attached to is restricted by its enchant keyword ability (see rule 502.45,
"Enchant").
Enchant Player, anyone? I know there's a lot of stuff in the Comp Rules (that's been there for a while) that just covers things that they may do in the future and have never done yet, but this seems like one they'd like to take full advantage of.