I'm hoping someone might be able to clarify what the “Update Input Landscape Data” checkbox does.
This option appears when you have a landscape input in your HDA:
The documentation says:
When exporting as Heightfield, you can use the Update Input Landscape Data option. When enabled, if the HDA outputs a heightfield that is similar in size to the input landscape, then that input landscape's data will be updated instead of creating a new landscape actor.
Which makes me think that it updates the existing landscape actor used as the input instead of creating a new one under the HDA in the outliner. However, this doesn't appear to be the actual behavior.
What is the effect/purpose of this checkbox and how is it utilized?
Update input landscape data does exactly what you think: when feeding a landscape to an hda via a landscape input, it tells the plugin to try and update the input landscape instead of creating a new landscape actor.
For that to work, it requires that your HDA outputs a Heightfield that has the same X/Y resolution (number of points) than your input landscape. If that's not the case, then a new landscape will be created.
After tinkering a little longer, I think my problem was that I was operating on Landscape Proxies. This functionality doesn't seem to work on that object type. But it works just as described with a regular landscape.