EDL is a failsafe, read-only boot mode hardcoded into the Qualcomm boot ROM (read-only memory). It is the lowest level of software on the device, below the bootloader, recovery, and even the primary boot partition.
If you have ever bricked an Android device, especially one powered by a Qualcomm Snapdragon processor, you may have plugged it into your computer only to see an unfamiliar device appear in Device Manager: QUSB_BULK_CID . qusb bulk cid
