Bip39 Checksum. At the start you need to store it Individually from the seed. Owning your seed and your passphrase in precisely the same place defeats the objective of aquiring a passphrase in the first place. No, there is not any technical limit to the length of the passphrase. However, https://techonpage.com/story4951809/bip39-generator-https-linktr-ee-bip39-how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-wallets