Field help (PSH1PRHFCY) 

Specify the site where the goods must be delivered by the supplier. It is initialized to the order site if the latter is also a warehouse.
For a purchase request or an order for a product managed in stock:

However when the request site is not declared as a warehouse in the site table and the ordered product is managed in stock, it is the first storage site associated with the legal company of the request site that is submitted.

From the Receiving site field, you can:

SEEWARNINGThe receipt site must belong to the same legal company as the requesting site for a purchase order or the order site in case of an order.

If you change the receiving site, a message is displayed to suggest you to run the search for the default supplier again: