This is a very useful layer space transform. Many effects in After Effects (like Bulge or Beam) only exist in 2D space, but sometimes you want to those effects to work within a 3D environment. The 2D TO 3D connector expression helps you bridge that gap by using the toComp method to tether your 2D effect point to a point on your target 3D layer, and keep it aligned properly with respect to the camera view.
Code Sample
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L1 = thisComp.layer(“your3DLayerName”);
L1.toComp(L1.transform.anchorPoint);