Mailing address : You have to give exactly the address where you should receive all the correspondence from CIC
Residential address : If the mailing address and residential address are different give the home address there.
Otherwise copy the same mailing address given already.
hi. i think you are referring to IMM0008, and not the background declaration form. residential address is the address where you presently reside. the mailing address is the address where you want to send all the documents mailed by the immigration.