The cross section used for weighting Monte Carlo events depends on how those events are thrown. There are two options in the Qweak Monte Carlo:

isotropy = 0: flat theta distribution, requires a modified weight for the cross section -> QweakSimUserPrimaryEvent::CrossSectionWeight
isotropy = 1: uniform spherical theta/phi distribution, use regular cross section -> QweakSimUserPrimaryEvent::CrossSection
else: not defined