I was reading some older postings regarding how to obtain the hours on a CTD. I ran through the steps and several other numbers came up. Does anyone know what they mean?

S-XXXX
I-XXXX
C-XXXX
Hr-175

The "hr" is self explanatory, but the others? As posted before this is the first diesel of any kind owned, just trying to figure out all of the little things so I can hang on to it for a while.