Item subsitutions, how does it work automatically?

I am looking for any documents or inputs of how item subsitubes are sourced automatically in PS 9.2. I set item subsitube usage as 'Manual and Automatic' at the set id level. However, i am unsure of what critiria that make the souricng job  to select subsitube items automatically instead of primary items when generating  PO or MSR.

Thanks