Two types of data can be transferred while using SwiftKey Keyboard.
First, anonymous usage stats are periodically sent back (much like the statistics you can see for yourself under Settings --> Stats) to help us understand how people use SwiftKey and how we can make it better. We get meta-level insights such as how many characters people have entered in total and how many keystrokes have been saved, but we don’t know anything about what you have been writing. If the app crashes, anonymous info about the crash will be sent to Google and to us.
Second, if you create a SwiftKey Account, various additional data can be transferred to our servers to help us provide these services. For example, if you have Backup & Sync enabled, you give explicit permission for your personal language model data to be stored on our servers and processed to ensure prediction consistency between your devices. If you elect to use the SwiftKey Account personalization services, you give explicit permission for our servers to process your personal message data from these services, in order to improve the word predictions that SwiftKey gives you.
All of this data can be destroyed at any time by erasing your SwiftKey Account data or deleting your SwiftKey Account. Options to do this are found under SwiftKey Settings --> Account.