SY - System fields and return codes in ABAP

Hi

I'm new to ABAP, where do I find what the meaning of the system field return codes are ?

example SY-SUBRC returns a number of codes 0, 2, 4, 8 etc what do they mean.

Is there a resource somewhere that provides this information for all the system fields.

I have searched high and low and have found little, they must be defined somwhere ?

cheers Hugh