A "useful proxy" is a way to represent or measure something indirectly, when direct measurement is difficult or impossible. It is a helpful substitute that helps us infer or estimate information about the thing we are interested in. Full definition
Consequently, tree - ring width and tree - ring density, both indicators of tree growth, serve as useful proxies for temperature. (skepticalscience.com)
Offsets in radiocarbon concentration within the ocean or between the ocean and the atmosphere are particularly useful proxies for a variety of studies. (eos.org)
This is way too simplistic on one hand, but a useful proxy for approximation. (politics.stackexchange.com)