How does Add Account Work
Bitcoin Improvement Proposal (BIP) 39 defines a formula forthe generation of a mnemonic sentence (also referred to as mnemonic words, seed phrase, recovery phrase, etc.)the generation of a seed from that mnemonic sentence. That seed is used to produce your private and public keys, but those details will be covered in the next post in this series.
BIP 32 is a specification for creating Hierarchical Deterministic wallets. They are wallets that, from a root it can generate multiple "child" private keys in a deterministic way. You only need to remember the "path" of the child key.For example hardware wallets use them from a single root you can generate separate keys for Bitcoin (with path m/44'/0'/0'/0) and Ethereum (path m/44'/60'/0'/0).
Add Account in Extension Wallet
Binance Chain extension wallet would use a similar way to generate keys as Ethereum, i.e. derive the private key using BIP32/BIP44 with HD prefix as "44'/60'/", which is the same as Ethereum's derivation path.
Whenever you click on “Add Account”, your seed is extended at the end by a counter value which makes it possible to automatically derive an unlimited number of new addresses in your extension wallet.