IMAGEX-WIN10x64 is an OS Image with flags :
F140-2 : Password encryption using a FIPS 140-2 compliant algorithm
Pe2Lm : installation via SMB share using a Windows PE 2.0 boot image
So it is Windows PE 2.0
IMAGEX64-WIN10 is an OS Image with flag :
F140-2 : Password encryption using a FIPS 140-2 compliant algorithm
Pe64Lm : installation via SMB share using a Windows PE x64 boot image
So it is Windows PE x64