Pretty self-explanatory, but admittedly most useful late-game. All of those steel longswords and such which those tribespeople drop? Yeah, chuck 'em in the smelter. However, I wouldn't want to smelt say any plasteel longswords in storage.
Granted, forbidding micro is a thing, but it'll get tedious after a while.
This mod adds a tool to designate specific items to smelt. https://steamcommunity.com/sharedfiles/filedetails/?id=1241579445&searchtext=smelt (https://steamcommunity.com/sharedfiles/filedetails/?id=1241579445&searchtext=smelt)
Hope it gets updated to 1.0, tho it'd be nice to see something like that in vanilla.