default / options keywords

[Ryan Schmidt]

Are the workings of the "default" and "options" keywords documented  
somewhere, or where in the code do I look to understand exactly how  
that's working?

It seems to be about late variable substitution, e.g. when the xcode  
portgroup says

default destroot.destdir    {DSTROOT="${destroot}"}

the curly brackets around the whole thing prevent variable  
substitution at the time the portgroup is loaded, and MacPorts plugs  
in the value of destroot into that string later, yes? It gives the  
port a chance to change destroot before it gets plugged into the string.

It also seems to be about being able to access portgroup variables  
without everyone having to "global" them first, e.g. you can just say  
"xcode.target something" in a portfile without having to first say  
"global xcode.target".

Is this all it is or is there more?