forward direction can still propagate into the next radial zone of the model and are therefore not lost from the radiation field [@Martin1979; @BFM; @AGN3]. In  this is approximated using an effective opacity $\alpha_{scat} (1-g)$, which is said to discount forward scattering. We refer to this as the extended extinction. Both extended and point-source extinctions are reported by the code, but the user should be aware that the PDR literature always uses the point-source extinction.

Line transfer
-------------

The treatment of line transfer is largely unchanged from F98. The escape probability formalism is used, using the framework given by, among others, @Irons.F78On-the-equality-of-the-mean-escape-probability, @FerlandElitzur1984, @Rybicki.G84Escape-probability-methods-pp-21-64, @Kalkofen.W84Methods-in-radiative-transfer, @Netzer1985, @Elitzur.M85Line-fluorescence-in-astrophysics, @Elitzur1986, @Kalkofen1987, @Ferland1992, and @Elitzur1992.

All permitted, and many forbidden, lines are transferred using a common approach. With this, processes such as line trapping and thermalization, pumping by the local radiation field, line destruction by background opacities such as photoelectric or grain absorption, are included for the $\sim 10^5 - 10^6$ atomic and molecular lines considered by the code. Line processes couple into the model atoms, which can go over to the correct thermodynamic limits when exposed to a blackbody [@Ferland1988]. The simulation is done self consistently, with feedback between various constituents taken 