C-peptide is formed when your pancreas produces insulin so it can be used to measure how much insulin you are producing.
They usually test C-peptide instead of insulin level because C-peptide is more stable.
It's one of the tests that can help determine if domeone is T1 or T2.
Interpreting C-peptide results isn't always straight forward.
If very low or zero, it's easy, most likely T1, can be T3C as well.
If very high, most likely T2, as T2's tend to overproduce insulin.
If somewhere in the middle, it can be either.
And to make it harder, the result should be interpreted alongside BG at the time of the blood draw.
A GP isn't likely to be able to correctly interpret C-peptide levels, in my country a GP can't even order this test. Needs an endo.