It's a multichannel stem filter with an independent LPF and HPF for each channel in a surround stem.
It has 2 main uses:
1. As a cleanup filter for your stem bus, right before you record.
It'll save you headroom on the main channels and filter the LFE at the same time.
2. As a bass management tool for your speaker outputs.
You can use the redirect buttons to take low frequencies out of the mains, and send it to the LFE.
Sound FX editors are a grubby bunch. Their pitch shifting, sub-thumping tricks can often lead to a lot of very low frequencies, or very high whines in places it doesn't belong. Don't trust them - they'll devour all the headroom on your center channel the moment your back is turned.
Stemcell will mop up any lingering DC offset or sub-harmonics caused by those meddlesome sound designers.
If you feel the need, gently roll off the super high end to stop anything sounding too 'adr' on the dialog bus. Or get even more drastic with that foley stem.
Stemcell lets you choose different settings for each channel so you can even use it to tone-down some over-excited surround speakers.
Stemcell offers a redirect button on each channel, taking LF from the main channels and sending it to the LFE.
In this mode it is effectively a monitor-chain bass-management processor, with tons of options for crossover frequency and slope.
Chuck it on your output master fader and give your LCR a bit of a break.