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Working with missing data- boolean masks and its aplicability

In the mission screen 370-2, I cannot understand why I am using the boolean mask and what is actually the role of a boolean mask in such a situation.

Is it the boolean mask required for imputation? When is the boolean mask necessary to use?

I cannot see how everything connects here and makes the logic work.

I would like very much if someone who understood can explain this to me and answer to my question.

Thank you very much!