Выдержка из оригинальной справки. (NDL Gamebryo 1.1)
NiParticleMeshesData objects contain the actual geometry data necessary to render an NiParticleMeshes object.
Set the number of active particles. Setting this value to be less than the number of vertices in the object will cause particles at the end of the vertex list to be ignored when transforming and rendering the primitive. If the active particle count is set to some usActive that is less than usVertices, then the last usVertices - usActive particles will be ignored. This function is useful to some applications that may wish to dynamically alter the number of particles in an effect at runtime. Note that this value is clamped to be less than or equal to the number of vertices in the primitive
