In the propensity score matching literature (Central Role of the Propensity Score by Rubin), the treatment effect estimand is referred to as the "Average Treatment Effect" (ATE). However, in some papers such as this one, the effect estimand from propensity score matching is referred to as the "Average Treatment Effect on Treated" (ATT). This is a bit confusing to me, as I thought it was the ATE that was being estimated. What is the correct estimand?