Passing vars from one .swf to another

I have a main .swf (a shell) that loads in external .swf files. I need to pass a variable from the main shell to an external .swf that is being loaded into a MC in the main timeline.

How do I do this?

loadVarsNum?
loadVars?

myVar = someVar (in the main timeline)
loadVarsNum (in the external file)