Compound device

A USB compound device contains an embedded USB hub and one or more non-removable USB devices. It may or may not have exposed downstream ports.
The internal USB hub may be a physical IC that connects to other ICs, or the hub and all functions may be implemented in software on a single IC. Compound devices have separate device addresses assigned to the hub and each downstream function, while composite devices have a single address. The hub's descriptors indicate whether it is part of a compound device or a regular hub with only removable devices.
This arrangement is used by wireless receivers for cordless keyboards and mice and sometimes for printers which have memory card slots.