Adding custom metadata - Houdini

As explained in the metadata section, you can attach custom metadata to almost every Atoms entity. 

You can add metadatas to your joints by adding new parameters on them. You can do it when exporting both a skeleton definition and an animation clip file (fbx or atomsclip).
Any parameter with a name starting with "atoms_" that you add to a joint in Maya or Houdini will be added as a new metadata to that joint in your skeleton definition or animation clip.

For instance, if you were to add a parameter named atoms_testAttribute, an integer metadata named "testAttribute" will be added to your joint in the skeleton definition.
You would then be able to access it in python or C++ during the simulation and perform some specific action for that joint only.

You could also animate that attribute and export it in your animation clips.