Why these two propositions have different requirements

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Proposition 2.18 is similar to 2.19.

Why we need $N$ flat in 2.19?

What's the difference between 2.18 and 2.19?

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In short, the difference is left-exactness, i.e. whether the left map is exact. Proposition 2.18 omits the $0 \longrightarrow M'$ map.