Windows PE is bootable from media such as CD-Rom,
USB flash drive, hard disks etc. The base version (without added drivers, packages or apps) requires only 512 MB of memory.
WinPE is an add-on to the ADK (Windows Assesment and Deployment Kit).
See also
WinRE (Windows Recovery Environment) is a minimal Windows operating system that is based on Windows PE.
WinRE includes several tools that an administrator can use to recover or reset Windows and diagnose Windows issues. If a device can't start the regular Windows operating system, the device tries to start WinRE.
Apprently, a configured WinRE is required for device encryption support