what types or classes of USB device will show in the system tray removal list?

I want to know what kind of USB device or USB class will show in the system tray "safe to remove device" list in Windows. I thought the OS put certain classes of USB device such as a storage or media device in the system tray, but I found my Aardvark I2C/SPI Host Adapter and a hub are also listed in the tray. Any comment is appreciated, thank you very much!