When XUMM generates an XRP ledger account, it will generate your account secret using Secret Numbers. Secret Numbers are less prone to user entry error than other secrets.
Not all XRP ledger clients / wallets support Secret Numbers. If you want to use your XUMM generated XRP ledger account with another client supporting the Family Seed secret format (
s...) there's good news: a Secret Numbers account secret can be turned into a Family Seed 🎉
This is also good news for the (very unlikely) scenario where XUMM would no longer exist: you can take your Secret Numbers, turn them into a Family Seed and use any other XRP ledger client.