As I just learned, Emmanuel's definition of APIs and SPIs is this:

• API for me is app developer facing APIs
• SPI are integrator facing APIs
• some have to be extended, some have to be used, I don't make a distinction

Whereas to me, an API is something which is used by others (think library), whereas an SPI is something which is implemented by others (think framework hooks). I like to make this distinction apparent in the packaging, as different rules apply for the evolvements of these things.