How is a track's 30sec defined that's obtained from preview_url (Spotify Web API; python)?

I am interested to use Audio raw dataset provided by Spotify Web API python, I wonder if the audio sample followed any rules to define its 30sec provided by preview_url.

Examples:

1. To extract the first 30sec of the track or,
2. To extract 30sec of the track after 1 mins or,
3. To extract 30sec from any part of the track between 1-3mins or,
4. Don't follow specific criteria?