Everything you'll ever need to know about GUID Partition Table (GPT)
https://www.ibm.com/developerworks/library/l-gpt/index.html]
(Make the most of large drives with GPT and Linux
Preparing for future disk storage with the GUID Partition Table)

Direct from the man himself, Roderick Smith the well known rEFInd GURU
http://www.rodsbooks.com/ and IBM also have info / links to all EFI, UEFI, secure boot etc. matters.

Also the IBM content can be downloaded as .pdf