We are going to talk about the optional
BIP39 passphrase.
If you have an hardware wallet which is
following the BIP39 standard, you can
decide to specify or not this optional
passphrase. If you decide not to specify
it, the mnemonic is stretched with a
salt consisting in a constant string.
This will lead to a 512 bits seed from
your given mnemonic.
Otherwise if you decide to specify this
passphrase, the stretching function will
produce a different seed from the same
mnemonic. In this way you can get several
wallets with the same mnemonic, just
changing the optional passphrase. This
may be useful for accounting reasons or
for management reasons and you can
get until 2 power 512 (2^512)
different wallets. This
feature makes possible to achieve also
security advantages. For example you can
save in a place your mnemonic and save
in a different place your passphrase so
both will be needed in order to decrypt
and unlock your funds.
This makes possible to have an
additional layer of security for your
funds. Another advantage is that if you
store a small amount of Bitcoin in a
wallet with a certain passphrase and you
store the maximum amount of Bitcoin in
a different wallet with a different pass
phrase, in case you for some reason are
forced to unlock your funds by physical
violence, for example, you can always
unlock the wallet with the minimum
amount of Bitcoin.
So there are several
advantages using this feature and many
wallets support this feature, so the use
of the optional BIP39 passphrase is
strongly recommended.