[Papervision3D] Displaying a preloader for a Collada (.DAE) file

[-a-]

_pf = new ParticleField(_pm,
 4000// number of particles,
 *null//alternative container*
 800//Field width,
 100//Field height,
 800//Field depth);

In the Effects branch what role does the alternative container play. If 
you do have an alternate container how does it link up with the rest of 
the logic flow.

a/