The UEFI Forum has released a new version of the PI spec. William’s blog entry has a copy of the relevent section of the release notes:
https://uefi.org/specifications
Click to access PI_Spec_1_7_final_Jan_2019.pdf
https://www.basicinputoutput.com/2019/02/uefi-forum-releases-pi-spec-17.html
Some input from Nikolaj: