The monoisotopic mass is calculated using only the most abundant isotope of each element (12C, 1H, 14N, 16O, 32S). It is the mass of the first peak in a resolved isotope pattern and is what high-resolution mass spectrometry measures.
The average mass (molecular weight) uses the natural abundance of all isotopes. It is the value used for weighing and for preparing solutions of known concentration. For a peptide of a few thousand daltons the two differ by about one to two daltons.
Product data normally give the average mass of the free peptide, not including any counter-ions.
See also
For in vitro laboratory research only. Not for human or veterinary use. This library describes chemistry, analytical methods and published laboratory research; it gives no guidance on use in people or animals.