In the paper about Region Proposal Networks (RPN) (Faster R-CNN), Ren et al. explain the mini-network which classifies and regresses canditate region proposals (so-called) anchors, and visualize it with the following diagram:
What exactly does the intermediate layer look like? What does the "d" refer to in "256-d"? Pictured here, it looks like it is a fully-connected layer with 256 hidden nodes, but in this video it is shown to be a two-dimensional activation map. Which is it?