APO availability check when creating the sales order

Hi

when creating the sale sorder in ECC there is a availability check happens in SCM APO. now if the material is not availble in certain plant this creates purchase req ( STR ) in ECC. I am debugging the sales order tcode va01 to understand how this happens. I came across a BAPI BAPI_APOATP_CHECK , which is present in SCM system but called remotely from va01 tcode. But not sure how the purchase req is created. is it with in this BAPI or some where else. can somebody help me on this.

thanks