Why TREC set two task: document ranking and passage ranking TREC is https://microsoft.github.io/TREC-2019-Deep-Learning/

I am new to text retrieval. Still can not understand why set the two similar task.

Thank you very much.
The idea behind the two tasks is to explore how document length affects the effectiveness of the different retrieval models.

Ellen Voorhees
TREC project manager 
NIST