Why might a certain product/service item from an estimate not appear on a purchase order created from that estimate?

The reason that a product or service item from an estimate may not appear on a purchase order created from that estimate is due to the specific setting associated with the item, which requires that the "I purchased this product/service from a vendor" option is checked. This setting indicates that the item is intended to be purchased from a vendor and allows it to be included in purchase orders. If this option is not checked, the system does not recognize the item as something to be ordered from a vendor, leading to its exclusion from the purchase order despite being present in the estimate.

In the context of the other options, if the estimate were not properly saved, it would typically result in the loss of the estimate data altogether. An out-of-stock product/service would not prevent it from appearing on the purchase order; it would just indicate that the item cannot be fulfilled immediately. Lastly, the need to create a purchase order first is misplaced because the purchase order is a tool to order items, not a prerequisite for their appearance based on the settings assigned to each item.