Tracking "Breaking News" Chyrons On Television News: What Is Considered A "Breaking" Story?

What are the kinds of news stories that television news considers a "breaking news" story? Using the TV AI Explorer, we can search the onscreen text of BBC, CNN, MSNBC and Fox News since the start of 2020 for the total daily seconds of airtime in which this phrase appears anywhere in the onscreen text, as seen in the streamgraph below:

Breaking stories are highly cyclic, trending towards weekdays and decreasing sharply on weekends. Using rolling average smoothing, the timeline below shows that BBC News has almost never used the phrase over the past two years, while Fox News uses it far less than its peers. CNN has been ramping down its use steadily since the start of the Ukraine invasion and it will be interesting to see to what degree it continues this reduction in line with its new editorial guidelines.