Compound

A compound is a container type which groups multiple .aux items into a single item, with an additional parent node to which they are tethered. Both the item data and the nodes of compounded items are contained within its compound item rather than having entries in the regular item and node arrays of the sector file.

The editor does not allow signs to be added to compounds, but signs without traffic rules can be added externally and the game will load them without issues.
Likewise, the editor requires a compound to consist of at least two items, but a compound of only one item, though somewhat pointless, is supported if created externally.
An equally pointless empty compound can be created by adding sub_mode_types[]: 40 to editor_data.sii and selecting Compound Model from the item type menu.

A compound which contains a .base item will crash the game.

Item ID: 0x28 (40)
Item file: aux

Structure

Field(s)	Description
struct kdop_item	See k-DOP item. Flags: Bit 0: Water reflection Bit 1: Ignore cut planes Bit 4: No collision Bit 5: No shadows Bit 6: No mirror reflection
u64 node_uid	UID of the compound's parent node.
u32 child_count array_struct children	Contains the items.
u32 child_node_count array_struct child_nodes	Contains the nodes of the items.