Create PO automatic in item category

Hi Experts..........

I am little bit confuse about P O Automatic in Item Category & Purchase Requisition Delivery schedule, basically If i check box of Purchase Req del sechdule and maintain Order type, Itm catg, Acct Assg Grp in VOV6 schedule line than this schedule line is use for third part sale, and a Purchase req is created in a background when we save the order. Here my Que is If this option is for Third party............than what is the need of "Create PO Automatic" in Item catg VOV7, as far as i think If I activate "Create PO Automatic" this check box than a Purchase Order ll also created in a back ground as soon as we save the Sales Order.......? second one for Third party sale "Create PO Automatic" is Mandatory or not.................?

third ........ for third party sale what is right option means Item catg with activation of "Create PO Automatic" or without activation of "Create PO Automatic".

Please clear me......................

thanks

Sarang