A change address is part of your vault, and the remaining balance of a partial withdrawal is sent there.
This is how bitcoin handles "change" when you spend a portion of your vault.
It's like if you ordered a pizza that cost $15 and you have two $10 bills in your wallet. The total transaction consists of $20, because you must pay with the two $10 bills (two UTXOs), and you will receive one $5 bill back (the change) as change back into your wallet.
The change is sent to a change address controlled by the same keys as your vault. You retain control of any remaining funds.
Learn: You can read more about change addresses in this bitcoin Stack Exchange article.
How Unchained constructs transactions
Over time, UTXOs (unspent transaction outputs) will accumulate in your vaults as you make bitcoin deposits. Every deposit is a UTXO with a predetermined bitcoin value. When you initiate a partial withdrawal, the Unchained platform automatically selects addresses with UTXOs that equal or exceed the amount of bitcoin needed. Because UTXOs represent discrete amounts of bitcoin, there will almost always be change left over. Any UTXOs not required for the transaction will be unaffected.
If you initiate a full withdrawal of all of your funds, every UTXO will be included in the transaction, and there will not be any change. This feature is helpful if you ever need to make a consolidation transaction.
Tip: You can verify this new address on your devices to confirm it belongs to your multisig quorum.