For plug-ins that will forever be internal to your site use the constructor that takes a single unsigned int parameter. The numeric range 19-26 (8 ids) has been reserved for such plug-ins.

Plug-ins that are intended to be shared between sites will need to have a globally unique id. The Autodesk Developer Network (ADN) will provide such id's. The MPxAnimCurveInterpolator class is the base class for user defined Anim Curve Interpolation Types. It allows for the creation and evaluation of customized animation curves, in addition to determination of the type ID and name of the curve. The evaluation of an animCurve between two of its keyframes is determined by interpolators (also known as "tangent types") at those keyframes. This class allows for the creation of custom tangent types.

Example: 2, 63 are registered by internal people @ Autodesk.