The pool's staking wallet address
This wallet maintains the magnitude, receives the proof of research awards, and interest payments.
Also the wallet handles the payouts to pool members.
If you send coins to the staking wallet, you will effectively "rain" coins on the hosts in the pool.
This is because payout amount is determined based on the wallet balance, minus the basis and interest
Also any project rain received into the staking wallet will be distributed to the hosts in the pool regardless of the project they are crunching on.