So let's say you have two proxy lists, e.g. 100 semi-dedicated and 20 private proxies. You want your tier 1 links built with private proxies, so they have less chance to get banned (e.g. if you use serengines, I think it is super critical). And then you use your semi dedicated ones for lower tiers. So this way you could set 100 semi-dedicated as public, 20 private as private in SER, but check "Use only private proxies for this project" option on tier 1 project. And on other projects tick "Use only public proxies for this project"
Would be great