A Monero wallet can hold several accounts, and each account any number of subaddresses. All of them come from your seed: restoring it brings them all back.
A subaddress is an address of your wallet that can’t be linked to your other ones on the blockchain. Give a different one to each person or service that pays you, and nobody can tell that two payments went to the same wallet. Create them on Receive with Create new address, and label each one.
Mainnet subaddresses start with an 8. A service that refuses them is out of date; ask them to support subaddresses.
Each account has its own balance, history and subaddresses: one for savings, one for spending, one for donations. Switching account is faster than opening another wallet, and having many accounts doesn’t slow down syncing.
A transaction spends from one account only. Accounts can’t be hidden or deleted. They keep your bookkeeping apart, but aren’t a privacy wall: someone holding addresses from two of your accounts can, with some effort, check whether they belong to the same wallet (the “Janus” attack). For identities that must never be linked, use separate wallets.
Every Monero wallet has one primary address, starting with a 4. It works for receiving, but Biscuit hides it on Receive to discourage reusing it. You may need it to create a view-only wallet, for solo mining, or for a service that doesn’t accept subaddresses.
Find it in Wallet › Keys, or on Receive: right-click the column titles › Show change address, and tick Show used addresses if it has received before.
After restoring from seed, Biscuit looks for payments on the first subaddresses of each account. If you used a very large number of them, raise the lookahead when restoring: Options on the seed page, then enter how many accounts and subaddresses to look ahead.