Atomic Wallet generates unique XRP address from the mnemonic seed phrase.
To use Ripple (XRP) in Atomic Wallet, you need to activate your XRP address and deposit at least 20 XRP on your account.
Once it’s done your address will be activated and you can manage XRP.
This 20 XRP will be frozen in the blockchain and you can’t withdraw them.
To learn more about 20 XRP rule please read this article.