Whirlpool is completely non custodial. The term "account" refers to an address space covered by your recovery words and passphrase. Within Whirlpool there are two areas of this address space that are segregated from the rest of the wallet to preserve privacy.
This account consists of all the bech32 UTXOs within your standard Samourai Wallet. UTXOs in this account have not been cycled through Whirlpool yet. You can receive deposits directly into this account or select UTXOs from this account to cycle.
This account consists of all the UTXOs that are prepared to cycle but are still pending. UTXOs will remain in Premix until they have one confirmation and they are selected in a Whirlpool cycle. These UTXOs cannot be spent yet but will have priority in any Whirlpool cycle over UTXOs in the Postmix Account.
This account consists of all the UTXOs that have completed at least one cycle. These UTXOs are available for spending from within Samourai Wallet or available to cycle again for greater privacy.