Thanks to BIP85, it is possible to generate infinite "child" seeds (e.g. 12 or 24 words seeds) from a single "parent" BIP39 seed. The child seeds can be easily reconstructed from the parent seed. However, the parent seed cannot be arrived at from the child seed.

Advantages: 

1. One single seed to remember (+ a few simple parameters for every child seed);

2. Security through isolation: child seeds are cryptographically independent of each other and the parent: exposure of one child seed doesn’t compromise the parent or other children. This allows you to use child seeds for less secure environments (e.g., a hot wallet) while keeping the parent seed offline in a highly secure location.

How to use them:

In iancoleman.io, after generating a master seed (e.g. 24 words + BIP39 passphrase), it's enough to tick the "Show BIP85" option and set (and note) the parameters:

• BIP85 Application (e.g. BIP39)

• BIP85 Mnemonic Language (e.g. English)

• BIP85 Mnemonic Length (e.g. 24)

• BIP85 Index: this is an arbitrary number (default value is 0). It can be any number; but each child must have a different number (e.g. 12345678).

These parameters, together with the parent seed, BIP39 passphrase and derivation path,  are enough to reproduce the child seed. This can be done in iancoleman.io or in a Coldcard hardware wallet.