Inner Boundary Conditions

Hello Gents,

Q:/ What is the reason for letting the inner boundary condition = (-1) when solving the radial flow of infinite form of diffusivity equation, and I would like to know what would happen if I didn't equate it with (-1).

As in the attached pic: https://i.imgur.com/AVesmHM.png

and here is the original pdf if you want: http://www.pe.tamu.edu/blasingame/da..._DimLssVar.pdf