Hello Jurgen,
the graphic for this one is http://help.sap.com/SAPHELP_470/Helpdata/EN/d3/17a1398010b740e10000000a11402f/content.htm
if you are using joint APO, then avaiibilty is carried out within APO, if differently APO then no availibilty is carried out.
if not using APO process is as described by SAP
http://help.sap.com/saphelp_afs64/helpdata/en/1e/fcbfacc48911d4b57c006094b9b9dd/content.htm
we have the process nearly working just need to know where vendor would be populated for external plant, this would be the same if using APO as vendor or plant must be assigned to a company code in order to create a Purchase order.
i have a OSS note raised now with SAP