There is no code. All RAM is the same. It is all manufactured by a few companies to about the same quality standards. The difference is that each chip has a specific set of specifications. Not all specifications are appropriate for all computers. All those manufacturers sell to the same set of retailers. So, the only thing really meaningless about RAM is the name brand.
I don't know the details of which specification work with which Macs. The easiest thing to do is just buy it from a reputable retailer that will guarantee the RAM will work in your Mac. Decent retailers will check those specifications and ensure it works before selling it. That is why RAM from Apple (which is identical to any other RAM) always works. If you don't want to pay Apple prices, I recommend
OWC Their prices are as cheap as you will find anywhere, and it will work.