There is no effective panel for editing the terrain sources elements.

The only available tools are the add/remove feature or the "Edit Terrain" tool with it's ""delete feature". All the above are extremely dependable on visual and clickable data.

There should be a panel with all the source features displayed where we could remove, deactivate/activate, change calculation priority, ...

I suggest all to be grouped by batch: all elements added in one command grouped.

Civil 3D has it this way and it works like a charm (see Surface Definition tab inside Surface Properties)

The way it is now causes a lot of bugs and terrains errors with no easy solution other then delete and recreate.

For example, after deleting a line that was part of the terrain, it's remais as a ghost for the terrain instead of being removed. The solution is to activate the breaklines display and using the "Edit Terrain - Delete Feature" tool. Thus adding a DTM edit to the terrain that can couse unwanted future behavior.

This also happens when using a line from a ref in the terrain. If the ref is detach, the line remais as a terrain breakline as the case above.