Autodesk supports a developer registration scheme to prevent Anim Curve Interpolation Types conflicts between different applications. Each registered developer registers with Autodesk one or more "Maya Tangent Types ID Block" to use in any Maya custom Anim Curve Interpolation Types saved into a Maya file. Of course, a developer may not choose to use an ID already registered by another developer.
Autodesk policies governing Maya Tangent Types ID registration. Autodesk reserves the right to:
Autodesk will take reasonable steps to register ID Blocks on a first come, first served basis.
Questions about registration and why you should register are answered here:
Why register a Maya Tangent Types ID?
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.
How many IDs do I need?
For plug-ins that will forever be internal to your site use the numeric range 19 to 26 (8 ids) which has been reserved for such plug-ins. In any other case, you need to reserve your own range which can contain up to 8 unique IDs.
Am I restricted to one range?
No, you can reserve as many blocks as you want at any time. Consider registering only what you need. 1 for each individual Tangent Type.