You can find these settings in the section 'Fast Input' in the menu 'Settings' > 'General'.
Billit automatically creates two unique email addresses for each file. Any documents you mail to theses addresses, end up automatically in 'Fast input'. Use the first address for files you don't want to split, and the second for documents you do want to split.
Switch on this option to automatically send documents to the tab 'Approve'. in 'Fast input'. Turn off the button and documents will automatically go to the tab 'To be processed'.
You can use the 'Approve' tab if you want to add an extra step to your approval flow and different people need to check your documents.