When would you use 'share' instead of 'proportion'?

You would use "share" when referring to a part of a whole, often in a qualitative or descriptive context, such as discussing the distribution of resources or responsibilities. "Proportion," on the other hand, is more precise and quantitative, typically used in mathematical or statistical contexts to express the ratio of one quantity to another. For example, you might say "a large share of the market" versus "the proportion of sales from a specific product."