[BR3W CITY]

What year/truck is it out of? I can't tell what its mounted on, is that on the power steering pump?

It looks like a diaphram sorta shape so maybe a charcoal cannister as Clyde said, or possibly a vacuum cannistor. It looks like there is a bunch of wiring going to it tho, instead of like vacuum lines or anything.

HD/Medium duty trucks have had so many cab configurations and special equipment like PTO's that its hard to diagram them or have a reference without knowing what it came out of. Trucks with dump beds or a refrigerated box or something like that have all sorts of weird wiring.