The Key Code (kc) for an individual data element should be something usable in a human readable data sheet format – and not a typical computer programming variable name. It should also be typeable from most keyboards. It must also be unique within the SIG's Data Dictionary.
For algabraic notation and hand written claculations (or even MathCad or Tex variable names) - the Key Code Symbol is used - however it will rarely be a unique symbol due to the small number of Greek letters and such.
For computer scripts and codes the mnemonic form should be used.