Can anyone help me understand when you're supposed to use the right r's( either little r or big R) when using Gauss's Law?

Electric Field inside and outside an insulating sphere (e.g. charge Plastic) inside Gaussian (D.ds = 2;0 = P.VG.V> surface R Dads cos(0) = Pv 3 4 TT 7.3 S Gaussian surface 4 TT Dods = Pv 3 3 r outside S 4TT 4 TT R3 DAn = Pv Dods = Pv 3 3 S E ATT D = Pup = &E EE( 4rr2 ) = Pv 3 R3 3 inside outside E = PVR3 R E = Pur R 3Er2 R 36 r