Will callback function gets executed only in error scenario - Passport SAML SSO

Question

I am learning about passport and working on SAML strategy, will this (err) => {} callback only run during case of error or it will run in all scenario, i.e successful, unsuccessful, error. Thanks for any suggestions.

passport.authenticate("saml", {
  failureRedirect: '/auth/login'
})(req, resp, (err) => {
  
});