A Tinderbox document is a saved as a single file, with a '.tbx' extension, and thus are often referred to as 'TBX' documents.
Tinderbox can create and save multiple TBX documents, limited only by storage space available. It can open multiple TBX documents at once, limited only by the host computer's resources. When Tinderbox is started, it will open any documents open when the app was last closed; this behaviour follows the host OS's System Preference/General 'Close windows when quitting an app', so may vary if that preference is changed.
Tinderbox also tracks recently opened documents on a sub-menu of the File menu. If multiple documents are open, the application's Window menu list all open windows from all documents. The latter listing does not indicate the document to which each document belongs.
Any non-default Document Settings and other customisations (link types, custom colours, user attributes, etc.) are stored in the TBX.
A Tinderbox TBX document stores its data in XML format; for some users with XML skills this can provide another way to interface with, or view, the data. Users altering TBX documents by editing the XML source should note that such work is not formally supported as an editing method, i.e. do not expect that Tech Support will be able to fix user mistakes editing source XML.
See more on the XML structure of TBX documents.