The role of CPU (in MHz) in Compute Offerings

[li jerry]

Hello all

What is the role of the CPU (in MHz) parameter in Compute Offerings?

I created two computing schemes CPU (in MHz) = 500, CPU (in MHz) = 2000

Then two VMs were created through these two computing schemes on top of 
xenserver 7.1.2.

I can't see the difference between these two programs through xencenter.