We recommend that the hardware device you use to sign and send transactions in the Casa App be used exclusively as a signing device for the Casa App. The reason is: Multisig with the Casa App is considered "seedless."
By "seedless," we mean that you should not write down or keep a copy of the seed phrase that is generated during the hardware device setup process.
"Why go seedless?"
The reason behind this is that having a seed phrase backup allows that key to be compromised without your knowledge.
Furthermore, your hardware wallet device can be secured with a PIN code, passphrase, or both - seed phrases cannot.
"But what if I lose or break my hardware wallet...?"
Seedless only works in a multisig setup because other keys exist to sign a transaction in the event that one of them is lost.
In the unlikely event that your hardware device is lost or damaged, you can still use the remaining keys in your multisig setup to spend funds.
That being said, if you have funds on the hardware wallet's standalone key, you DO need to retain the seed phrase, since that is your only backup should something happen to the hardware wallet. It's for this reason that we recommend that you use the hardware wallet device exclusively with Casa.
"What if I don't want to go seedless?"
While a seed phrase backup can be an additional point of compromise, and is not protected with a PIN code or passphrase like a hardware wallet is, it's ultimately up to you to decide if you want to retain the seed phrase or not.
There is no requirement that you "go seedless" when setting up multisig with Casa.
Should something happen to your hardware wallet, retaining a copy of the seed phrase would allow you to restore the signing key on a new device. In this scenario, no changes would need to be made to your keyset.
If you do decide to retain your seed phrase, make sure that it's securely protected. Keeping the seed phrase sealed in a way that is tamper evident is advised.
"What if I've already set up my hardware device with a seed phrase?"
Our recommendation is to use a seedless multisig setup. If you've already written down and stored your seed phrase, and added it to Casa, it's recommended that you rotate that device out of your keyset and replace it with a device that has no backup seed phrase.