charcoal cannister

i guess thats the correct name. got a 72 duster and 76 duster, both /6. i've had very few new enough to have this cannister and was usually canned anyway. how does this beast work? vacuum lines run to it, ....on the 72 a fuel vapor or "overflow"? line goes there. (whichever it is),,,, on the 76 fuel line there and then to vapor return line that goes to tank. if i remember if ya look at bottom you see some black "stuff". if carb float sticks and causes it to flood (holly 1 bbl) , i figure it gives a place for the gas to go? or is vapor only going to come out to this thing, why does vacuum lines go there....
i do appreciate any info,i will say in advance, anyone wanting to imply as to my dumbassness, go ahead, i'm 65 and have heard it before, especially when deserved??? LOL