What is the FME Plug-In SDK

From fmepedia

Q) What is the FME Plug-In SDK?

A) The Plug-In SDK is a programming tool that you can use to develop support for a new format for FME; usually your own corporate format, but I guess there's nothing to stop you creating a plug-in for any data format you need to use.

NB: Although FME Objects is also an SDK, the two SDKs are entirely different products.