.pxe is an image designed to be chain loaded, unloading both the underlying PXE and UNDI code sections.

.kpxe is a PXE image that keeps UNDI loaded and unloads PXE

.kkpxe is a PXE image that keeps PXE+UNDI loaded and return to PXE (instead of int 18h). From here

.dsk is the beginning of an image for a floppy disk

.sdsk is the beginning of an image for a “1.44M” (1440 kiB) floppy disk with a FAT file system and the Syslinux boot loader

.iso is an image of a CD using ISOLINUX to load a .lkrn

.liso is an image of a CD using floppy emulation mode containing a .dsk

.lkrn is a binary that can be loaded by any boot loader which can load a Linux kernel

.usb

.rom is an image that can be burned directly to a PXE ROM. Most images in this format are named for the PCI vendor/device ID (ie 10b79055.rom)

.hrom