It's the D|S SDK, as in Software Developement Kit.

It gives you access to the internals and code that runs D|S so that you can write plugins for D|S. Like eg if you wanted you could code a plugin that recreates the flocking behavior of birds and add it to D|S. The SDK would give you a library of functions to read various parts of the D|S scene, change it to your needs and finally send it back to the scene. And if you thought it would be a popular tool, you could also sell it.

I wish poser had something similar.