A quick test suggests its the currentTiddler, or what we could call the most recently open.
The title is updated in $:/HistoryList in the field current-tiddler
This is set according to the history and story list in use, but this is by default defined in $:/core/ui/PageTemplate/story
<$list filter="[list[$:/StoryList]]" history="$:/HistoryList" template={{$:/config/ui/ViewTemplate}} editTemplate={{$:/config/ui/EditTemplate}} storyview={{$:/view}} emptyMessage={{$:/config/EmptyStoryMessage}}/>
Regards
Tones