I'm in the process of putting together a new machine based on the Asus Prime Z370-A motherboard and need DDR4-2666 RAM. For 32 gigs via 2 16 gig modules at DDR4-2666 speed, your tool points me to http://www.crucial.com/usa/en/prime-z370-a/CT10998027 (CT10998027). It's currently $369.99 and out of stock.
I have found another web site that has in stock Crucial RAM w/the identical looking specs but with part number of CT2K16G4DFD8266. I found that at http://www.crucial.com/usa/en/ct2k16g4dfd8266. The specs look the same as CT10998027. The price is the same at $369.99 and it is also out of stock.
@cwerdna Thanks for your message. The two parts are basically the same. When you order online by system information, you are shown what we refer to as an “external” part number, in your case number CT10998027. It is a system specific number and is mapped exclusively to the ASUS PRIME Z370-A. The longer part number, CT2K16G4DFD8266, is the one that would you see printed on your module, and that is the actual module part number, or the "base" part number.
There are other specifications to memory than the basic ones published on the site, and they sometimes requires us to filter out a certain build of a part for a specific system. However, in your case it does look like the memory will be compatible with your system if you order using either link. Please let us know if you have any other questions or concerns.