The Casa app uses an HD (hierarchical deterministic) key, which generates a new address with each transaction that's made. Though we generally recommend not reusing addresses, any previous addresses that you've used for that keyset can be used again to receive funds, even though your Casa app is showing you a different deposit address.
Keep in mind that if you ever transfer funds to a new keyset in your Casa app--which you may do if, for example, you replace a hardware wallet device in the future--you should not send to any old addresses anymore, since they would then be part of your old vault.
Can I send from a specific UTXO?
Yes! In the send flow, you have the option to choose the address you want to send from. Simply tap "Send From" at the top of the send window, then tap the address or addresses you want your transaction to send from.
What happens to my UTXOs if I transfer to a new vault?
If you transfer to a new vault (when rotating a key or making a recovery transaction) using Guided Recovery, funds from the different addresses in the old vault (UTXOs) are swept and consolidated to a single address in the new vault. If you do not want to combine them, you have the option to send each transaction manually using Manual Recovery.