Self-powered?

Associated descriptor fields: bmAttributes (in the Configuration Descriptor)

Check this box if the device is powered, even partially, by any power source other than VBUS in the USB cable, at any time while the device is physically connected to the USB host. Only leave this box unchecked if there is NO power source for this device other than VBUS when connected to the host.

This assists the host in budgeting the power it provides to attached USB devices.