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

I reopened caused I dont even know what are the reasons this post is closed. It would be good to give some guidance or educate abit before removing any question I guess.

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 of what I attempt to ask:

1. Do you extract the 30sec of the track from the first 30 sec or,
2. Do you extract the 30sec of the track after 1 mins or,
3. Do you extract the 30sec of the track between 1-3mins or,
4. Do you extract the 30sec of the track randomly or others?