Here's a screenshot:
This is from Evaluating Continuous Training Programs Using Generalized Propensity Scores by Jochen Cluve, Hilmar Schneider, Arne Ulendorff and Zhong Zhao. I understand from the text that whether the treatment is at a value t is independent of the covariates X when the generalized propensity score at (t, X) is fixed. But the notation is unfamiliar. It's also in the original GPS paper.
What does it mean? What would 1{T = t} mean? Was it invented specifically for describing GPS?