A change address is part of your vault, and the remaining balance of a partial withdrawal is sent there.
A bitcoin wallet balance consists of UTXOs, or “chunks of bitcoin” received from each deposit. During a withdrawal out of a bitcoin wallet, UTXOs must be chosen to be spent. It’s like choosing which dollar bills to spend from a physical wallet holding cash.
Unless you are spending your entire wallet balance, the amount you are withdrawing is unlikely to match your UTXOs exactly. In this case, you must receive change back. It’s similar to using two $10 bills to pay for something that costs $15. You will receive $5 back as change.
In an Unchained vault, you will often see change involved during a withdrawal transaction. It is being sent to your change address, which is part of your vault. It is controlled by the same keys as your other vault addresses, so you retain control of any remaining funds.
Tip: You can verify your change address on your hardware wallet to confirm it belongs to your multisig quorum.
If you’re curious about how Unchained selects UTXOs automatically for a withdrawal transaction within your vault, you can learn more here.