A complex item is a resource that has a definite "type", but has sub-varieties.
In list inventories, they appear as "type": "subtype".
In stash/take inventories, they appear as "type...", with a submenu of relevant types.
Current Complex Items:
Items made on an anvil or in a Glory Hole are also complex items. They are listed by their quality; in the case of very high quality blacksmithed items, by name.
Note that thermometers are not complex items.