Hi mster,
Did you also see this web cast: http://kentweare.blogspot.com/2009/07/biztalk-light-and-easy-viewing-series.html
Use IDocs for Asyncronous(one-way) messaging and use RFCs for Syncronous (two-way) messaging. The way you generate the schemas is pretty much the same. You just need to know which patern the interface will use.
Kent