I'm trying to understand the notation I've found in a paper which doesn't seem to be defined within the text. I can't quite recreate it in MathJax but it looks something like: $$\propto \Bbb{1}(\overset{\Delta}{\pi}(f^a_t), u^a_t)$$ In the paper the 1 is blackboard bold style. I'm not sure what the significance of the 1 is. At first I thought it was an indicator function but there's no subscript and it takes two parameters.
Here's a pic of the original with more context. It's from an RL paper if that is relevant.
How do I interpret this 1 function?