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Hi there!
Thanks for reaching out :)
From what I understand, similarity ratio is used more in geometry. For example, two triangles could have a similarity ratio of 1:3. One triangle is 3 times bigger than the first, but its still the same triangle, which would make it similar.
A proportionality ratio, on the other hand, could be used in any other situation. It represents a relation between two quantities!
To learn more on that topic, it's right over here:
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Ariane