what does "SPI class" mean? (in Spring MVC documentation) [duplicate]

This question already has an answer here:

• difference between SPI and API? 8 answers

The Spring documentation of SimpleControllerHandlerAdapter ( http://ift.tt/1IXEIZ6 ) says:

This is an SPI class, not used directly by application code.

What does SPI mean? What this abbreviation stand for?