What is the difference between purity and net peptide content?

They answer different questions. HPLC purity is the percentage of the target peak among all UV-absorbing peaks — how clean the peptide fraction is. Net peptide content (measured by amino-acid or nitrogen analysis) is how much of the vial's mass is actually peptide, versus water, salts, and counter-ions such as trifluoroacetic acid (TFA) left from synthesis. The two can diverge sharply: a vial can read 99% pure yet be only, say, ~80% peptide by mass, with the rest bound water and salts. That matters whenever material is weighed or quantified. A thorough COA reports both, plus mass-spec identity — purity, net content, and identity together give the full picture.