What does tracking apply to?

When I specify a track word, does the Twitter system then apply this filter to the updates posted ONLY by the people I follow or does Twitter use the word being tracked to retrieve the updates posted by anyone who is a Twitter user?
I hope that it's the former. In other words, I hope that tracking allows you to be notified about updates posted by people you follow BUT ONLY when they post something with the track word.
If it it's the latter then it loses its value because you could get an update from someone you do not want to follow and have no interest in.
 
silly I’m amused

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