When to not using SI symbols for units?

In the book What If?, the author usually write the unit as kilometer, atmosphere, megawatt instead of km, atm, MW. However he does use the symbols in the drawings:

In one line both mm and millimeter are even used together:

This makes me confused because I can't find a reason to not using the SI symbols. I guess the reason is similar to writing numbers in words when doing so doesn't look silly for better reading flow feeling. But in the above example, I can't explain why the first number should use mm while the second one should use millimeter.

Is there a general rule for writing out the unit instead of its SI symbol?

posted over 7 years ago

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Ooker‭

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