By selecting the Registration requirements check box, the user must register the receipt before inventory is updated through the product receipt on a purchase order. Register a product to record the item’s physical presence at the company. These products are a part of the physical warehouse on-hand. However, the cost will not be updated until they are physically updated with a product receipt update. Generally, this option is used with the Warehouse management functionality.
It is mandatory if you enable Registration requirements check-box on Item’s Item model group setup, if not then not mandatory.
I don’t think it is possible to perform multiple registrations for multiple POs at a time. You might need some customization around the common form used for handling multiple POs. For example: Arrival overview.