With autobackups, controlled via Preferences, a backup of the current TBX is placed in the Tinderbox backup folder each time the TBX is opened. Up to 4 backups are stored per TBX. Note this is not an in-session autosave, but more akin to the OS' Time Machine system.
The backed up files are stored in the 'backups' sub-folder of the app's support folder. Once 4 backups of a given TBX file exist the oldest copy is overwritten by any newly created back-up.
Newly created TBX documents that have never been saved aren't backed up - this is useful as quick temporary document don't end up bulking out the backups folder.
Those who regularly access a wide range of saved TBXs should consider turning the feature off at app level and then turn for for individual documents as required. Those who have only a few TBXs can likely safely leave the feature on.
There is no harm in manually deleting any/all TBXs from the back-ups folder as the actual document still resides in its normal location. Those leaving the feature on and using a lot of files might make a note to check the backups folder from time to time and weed as required (e.g. to recover disk space, reduce the size of OS-level backups, etc.).