I'd like to know when event
sales_quote_item_set_product is triggered, please?
The event is triggered in
Mage_Sales_Model_Quote_Item class, method
setProduct($product). This method sets the product for quote item and dispatches the
sales_quote_item_set_product event along with product and quote item informations.
Mage::dispatchEvent('sales_quote_item_set_product', array( 'product' => $product, 'quote_item' => $this ));
When you set a product to quote. File where the event is dispatched