The answer: SOFTIMAGE Plug-in Description Language
The question: What does SPDL stand for?
The origins of SPDL go back to at least 1997. The original SPDL parser was written with BISON and FLEX (GNU versions of yacc and lex), and at one point I think there where three different parsers, two based on BISON/FLEX, and one C++/COM (that was back in the early Twister/Sumatra/DS days). I remember going through the BISON/FLEX files in order to write up a SPDL syntax doc…fun times!
FUN FACT: There are 1,511 spdl files in the Softimage 2013 SP1 installation folder.