I don't remember it clearly after so many years. This relocation type is used in Blackfin FDPIC ABI. Users usually don't need to understand how it works since toolchain will take care of it. If you are going to work on it, I would suggest carefully reading the document in your post several times. There are other processors which use FDPIC ABI.
ftp://ftp.redhat.com/redhat/gnupro/FRV/FDPIC-ABI.txt
Joseph S. Myers - Draft SH uClinux FDPIC ABI
Hope they will also help you understand FDPIC.