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?