Borusyak, 2021 has a sentence
We show that this estimator takes a particularly transparent form under unrestricted treatment effect heterogeneity, while our construction also yields efficiency when some restrictions on treatment effects are imposed.
I am wondering what does under unrestricted treatment effect heterogeneity mean? I translate it to that "when the treatment effect heterogeneity is not restricted", and it also means that this situation works in both cases "treatment effect heterogeneity" and "treatment effect homogeneity". Is it a correct thought?
I did ask this in the English Learner but they said this content is beyond the English language question.
The answer from English learner's expert is
However, I am more inclined to interpret it as "the transparency of our estimator when there are no restrictions is greater than the transparency of other people's estimator when there are no restrictions" and "the efficiency of our construction when there are restrictions is greater than the efficiency of other people's construction when there are restrictions."