The third input is the group “modulus”, see Miles’ comment here: Why nf is defined modulus -1 in sitetype "fermion" "electron" - #2 by miles
qnnames are just labels but they are rigorously related to conserved global abelian symmetries of the states/operators. Those lines describe how the space is split up, so that all operations are block diagonal within that space. If total Sz is conserved and total fermion number is not conserved then fermion parity is conserved.
Perhaps check out the ITensor paper section 8:
https://scipost.org/SciPostPhysCodeb.4/pdf
and the references in that section