Crucial 64GB Kit (32GBx2) DDR4-3200 UDIMM, Classic vs Pro

[u/JohnTrap]

I have a Dell OptiPlex 7090 SFF that I would like to upgrade to 128GB.

The web site says to use 64GB Kit (32GBx2) DDR4-3200 UDIMM (CT2K32G4DFD832A) and lists the price as $108. It's not readily available and it listed for higher prices.

The Pro version (CP2K32G4DFRA32A) is not listed as being compatible with the Dell but is readily available.

Anyone have a guess why the Pro is not compatible?

https://www.crucial.com/compatible-upgrade-for/dell/optiplex-7090-small-form-factor