For booting before Bootp.
PXE is typically used for Maintenance or other types of "non-usercontrolled" times.
PXE is used by Norton's Ghost server which distributes the images as needed.
Novell has ZENworks that can install things on the machien without even booting the hard disk for real. It just writes striaght to the disk. The PXE environ for Novell is a Small Linux Distro that is either on the HD or from TFTP.