The USB mass storage device class (also known as USB MSC or UMS) is a set of computing communications protocols defined by the USB Implementers Forum that makes a USB device accessible to a host computing device and enables file transfers between the host and the USB device. To a host, the USB device acts as an external hard drive; the protocol set interfaces with a number of storage devices.
More information on Wikipedia.
usb_host_class_mass_storage
Source code: src/drivers/usb/host/class/mass_storage.h, src/drivers/usb/host/class/mass_storage.c
drivers/usb/host/class/mass_storage.h