Descendant vs //

I would like to ask the following question:

according to xpath's specification these are not generally equal.

Except the case of element position
i.e.
//para[1] and /descendant::para[1]
where there is a difference (as stated in the specification), is there any
other case where they are not equal;
i.e
//a[@b=3] , /descendant::a[@b=3]

//a/b, /descendant::a/b

Thank you,
Ioanna.