Rob
Ansys Employee

Source terms are assigned to the CELL ZONE, and are mass/heat/momentum etc per unit volume, so some care is needed to make sure you add the correct amount of material. 

No need to alter the alternative treatment, as to what it does, check the manual  https://ansyshelp.ansys.com/account/Secured?returnurl=/Views/Secured/corp/v221/en/flu_ug/flu_ug_sec_mphase_massxfr.html%23flu_ug_sec_mphase_massxfr_altern_e