When the nails have grown, leaving a gap in z3, but the Whites are still in a good position (ie - you can't see the natural free edge below the smile) you would need to infill.
For an infill, I take down some length if the client wants me too - thin the new free edge, which naturally moves the apex back to where it should be - and then fill z3, blending into z2.
An infill is usually done 2 weeks after a new set.
I consider a rebalance to be when the White needs to be moved back (ie when the natural free edge is visable.)
So I do the same as above, but remove enough product to enable me to replace the white without creating a bulky nail.
It is usually necessary to rebalance 4 weeks after a new set.
So, it would 'normally' go:
Full set
Infill (2 weeks later)
Rebalance (2 weeks later)
Infill (2 weeks later)
Rebalance (2 weeks later) and so on.
All clients are different, some go longer...
Hope that made sense!