The first input is the height field to project into, the second input is the geometry to project.
The node sends rays from the height field to the surface and (if it hits) uses the distance between the points to modify the height field value.
Usually you will use this to import 3D geometry such as buildings or pre-made polygonal mountains into the height field, using the defaults (Hit farthest on, Combine method set to "Maximum"). You can reverse those settings (Hit farthest off, Combine method set to "Minimum") to lower the geometry shape instead of raising it. (It might also be possible to create weird or interesting effects using other settings.)
This node projects geometry into a height field as 3D height. To project the outline of geometry into a mask field as 2D, use Height Field Mask by Object.
The name of the layer to project the 3D geometry into, usually
When projecting, record the ray hit farthest collision surface instead of the closest. This defaults to on. To create valleys instead of mountains, turn this off and set Combine Method to "Minimum".
Combine with Existing
How to combine this mask with any existing mask in the input.
Clear the existing mask and replace it with the new mask.
Add the values in this mask to any existing mask.
Multiply the values of the old mask by the values in this mask. This might be useful to "scale" existing values while leaving empty areas alone.
Set the mask values to the maximum of the old mask and this mask.
Set the mask values to the minimum of the old mask and this mask.
Max Ray Dist
The maximum distance from the original position of the point the rays will look at. The node will not detect intersections outside this range. If the geometry is far from the height field in 3D space, you may need to increase this. If Combine method is "Minimum", any points farther from the surface than this will not be moved.