What is this and does it need to be hooked to a vacuum source

[maxreps]

Looking around behind the glove compartment area as I started putting my dash together I saw the heater/ac box with it's vacuum lines and manual controls... I followed a vacuum line that went through the firewall to see where it went and it was connected to the small round canister near on the firewall above the heater core lines. It also had a larger diameter hose coming out of it which I traced down to the A arm, dangling and unconnected.
I am assuming it's a vacuum canister for the heat/ac stuff in the cab but wanted to make sure.
Also, am I right in saying the line/hose coming out of the canister needs to be hooked to an engine vacuum source?