[Ifeffit] Units for density in Athena's Booth self-absorption correction?

Dear XAFS community, I am trying to use the Booth algorithm to correct for self-absorption in Athena, and one of the input fields is for the density of the material. What units should be used for the density in that field? (I could not find the answer in Section 9.9 of the Athena documentation on self-absorption approximations.) Many thanks, John