The .bak file is actually a Cubase project (.cpr) file and is written at the intervals set in the Preferences. If a crash occurs, all changes in the project file since the last automatic backup are not recorded in the .bak file.
Rendered files are audio files in the audio format set in the project settings. They are created immediately during rendering (0 bytes) and then written with audio data (x bytes). If a crash occurs between creating and writing, 0-byte files will remain in the folder.