In my yard, cast iron goes in one dumpster, regular iron goes into another, and shred goes into another.

Cast iron are things like bath tubs, sinks, heating radiators, fire place doors, brake drums, ect. you can tell its cast if you take a sledge hammer to it. You can break most cast iron with a few swings and the brake will look grainy.

Regular iron is any steel 1/2" thick or larger. This is usually the stuff that the shreder cannot shred. This is stuff like engine blocks, brake rotors, prepaired farm machinery, structural beams, ect...

Tin is anything that is smaller than regular iron, this will consist of most of your scrap.