What is a “Sub” or “Cross” number? A “Sub” or “Cross” number, is a genuine manufacturer replacement part. Manufacturers such as IBM, HP and DELL all have part numbers that can be replaced by another part number and will work identically. For Instance; Dell has Part Number: W5184 as a power supply for one of their systems, but they also make Part Number: U5425 for the exact same system and has all the exact same specifications. So if you were to be looking for Part Number: W5184, we may say “Unfortunately we do not have that exact part number in stock, but we do have “sub” number U5425 Qty 1 in stock for $25”. We only offer “Sub” or “Cross” numbers if the parts are exact replacements of each other.