This section looks at the syntax used for devices and file paths. The following information has been copied from the help topic FORMATS (run command bcdedit.exe /? FORMATS [enter])
To expand on this, specifying BOOT will force the boot device/partition to be used - this will be the device from which BOOTMGR is loaded. Unlike the other options (e.g. specifying a drive letter) specifying the device as boot will not lock the entry to a particular device by forcing the search for a disk signature and partition offset value. Setting the device as boot is particularly useful when cloning a system.
Using a drive letter will create an entry in the BCD store containing disk signature and partition offset information.
A drive letter can be substituted with a \Device\HarddiskVolume# entry if the drive has not been mounted - where # is a numeric value starting from 1. This is not recommended if multiple devices are attached to the system as it can be complicated to predict the volume number. Based upon my limited tests the volume number appears to be allocated in the following manner -
Assuming we wanted to add a menu entry to boot WinPE from the first partition on hard disk zero (partition is mounted as C: drive and is the boot partition containing BOOTMGR and the BCD store) we could use any of the following commands -