UEFI Platform Initialization (PI) describes the boot execution phases to encompass UEFI supported protocols and services. PI evolved from the original Intel® Platform Innovation Firmware for EFI Specification. While UEFI primarily focuses on the interface between firmware and the operating system, PI focuses on interfaces between low-level firmware components.
The PLATFORM INITIALIZATION WORK GROUP (PIWG) of the UEFI Forum manages specification development.
"The PEI and DXE specifications authored by Intel and other related specifications as mutually agreed to by a majority of the work group. Implementations of these specifications are intended to serve as firmware for initializing a computer system and are intended to be implementations below the interface layer presented by the UEFI Specification. Implementations of PEI and DXE would not be required for UEFI compliance." - http://uefi.org/workinggroups
More information: UEFI & PI FAQ | PI Boot Flow | PI Boot Phases
