Type Members and Covariance

I guess, "type variance annotations" (+ and -) cannot be applied to "type members". In order to explain it to myself I considered the following example

abstract class Box {type T; val element: T}

Now if I want to create class StringBox I have to extend Box:

class StringBox extends Box { type T = String; override val element = ""}

So I can say that Box is naturally covariant in type T. In other words, the classes with type members are covariant in those types.

Does it make sense ?
How would you describe the relationship between type members and type variance ?