Signed, Sent, Delivered

The xumm platform allows developers and app users to "meet up". While users can simply use the xumm application to track their accounts, balances and transactions, the true power of xumm is the platform available for developers and app users.

XRP transactions are usually user initiated: open your wallet, enter the destination, amount, etc. and then you submit your transaction. In retail / e-commerce (and many other) scenarios, by "reversing" this process, the payment flow will become less prone to mistakes and much more user friendly.
XUMM App

XUMM API for developers

An XRPL transaction "template" can be posted to the xumm platform API. Your sign request will be stored (called a payload), so the xumm app (iOS/Android) user can open the sign request (by scanning a QR, deeplink or push notification) and resolve (reject/sign) on their own device.

When a user signs your sign request, he or she is then able to trust your application. In this case an app specific user token will be generated for your application, and future payloads will be delivered straight to the end user with a push notification.

As the xumm platform offers different status update options with all the information you need to verify the signed transaction on the XRP ledger, you can, but don't have to rely on transaction callback information or transaction submission by xumm.
Read the docs Developer Console
XUMM App
XUMM App
XRPL Labs