Setting the Default Source
The default source indicates the source (or "content set") your book is saved with. So if you save your book in English, you'll want to set the default source to English.
The default source is stored as a book property, tts_defaultSource. It is used to check if a source change is required when your book opens. When a user opens your application, the ToolBook Translation System (TTS) will compare the current source to the default source. Only if they differ will TTS swap sources (i.e. load content from a different source).
To select a default source, simply click on one from the list of defined sources.