implicit declaration?

This:

    <p:identity>
        <p:input port="source"/>
    </p:identity>

is semantically equal to this:

    <p:identity/>

Specifically, I can declare the input port explicitly or implicitly.  As I
read through the WD, what is it that tells me that a port can be declared
implicitly or must be declared explicitly?  Is it whether it's a primary
port?

TIA,

James Garriss
http://garriss.blogspot.com