someone explain the diff to me. i am looking for a dc power supply for a car audio show board and i see some supplies with Coarse and Fine dials for voltage and current. What is the difference?

like this peice for example

 

its for fine tunning. you use the coarse on to get it at say 12v and the fine one to bring it too 12.5v or 12.3

 

what does that cost to get one

 

I'm interested as well, car audio in home during winter months = bliss.