What does MD545FD/A model of iPad means? Usually there is a model number with AXXXX. What does MD545FD/A mean? At Identify your iPad model you can see there are avery of them with AXXX.
Looking on the box my MacBook Pro came in the Model No: is designated as e.g., A1398 and the Part Number: is in the form of AlphaNumeric/A as is with the part number you've shown.  So with an iPad I'd assume it's the same, a part number, as there would be no reason to have different inventory control systems.  If you Google "MD545FD/A", it shows as an Apple iPad mini Wi-Fi + Cellular 64GB.