What format are the ThinkBook backup files?

They are just XML with the .tbb extension so ThinkBook knows to treat them as backups.

You can open them with a text editor see their contents. With all your data in an open format you know you can always access it, even if you don't have ThinkBook handy.

You CAN edit these files in you absolutely need to, but if you want to work outside of ThinkBook we recommend exporting your notes as text, then re-importing them. It's a much more friendly format.