Which userexits is used in PO automation ?

How PO automation work ? I have made all required seeting in vender master, info rec, source list and MM purchase tab. Have created Me51 and made sure that source has been determined. But Req is not converting in PO.

I looked into some forum and picked up that we need to change setting in ML91, but not sure as ML91 is for service orders whereas we trade material.

Please advise,