Answer: 

Split Receipting becomes accessible when you adjust the Receipt Quantity of a product to be less than the Order Quantity.

By default, the Receipt Quantity will match the Order Quantity. When this is the case, the only option available is to Confirm Receipt as there is no quantity remaining unallocated to a receipt. 

If you change the Receipt Quantity to be less than the Ordered Quantity, the option to Split Receipt will now appear.

For more information on creating Split Receipts, see Split Receipting.