Is there a meaning of in names of AWS instance type prefixes? or it just random?

You will find these prefixes in aws documentation over various instances types: 'a', 'm', 't', 'r', 'c', 'u', 'x', 'd', 'i', 'f', 'g'.

I can only assume

  C stands for Compute
  R stands for RAM 
  G stands for GPU or Graphics
  I stands for I/O 
  M stands for medium??

I wonder if other prefixes have some kind of meaning too.