sets of PROTOS designed to work together may conform to a common interface in order to pass data to each other through agreed upon channels.
e.g., a Group{} PROTO built to manage its children as a particle system might only enforce that they have position and color fields. any other characteristics (geometry, animation, etc.) can be left up to the children to handle.
this allows many different implementations of objects that can all be managed by the particle system PROTO, because they conform to the abstract interface of having position and color fields.
[wrl] [simple particle system] [child type a]