Difference between Terrain Height and Length

I read out the document of terrain in documenations

I cannot find the difference between Terrain Height and Lenght. Please help.

The Width and Length are the size of the terrain. You will see the giant flat plane change size when you change those. Height is an entirely different beast.

Clearly, new Terrain has no height – you are going to paint that. It isn’t the starting height, since that’s what Flatten Heightmap allows you to do. Turns out that Height is the maximum height it will allow you to paint.

That seems silly – why not always max it out? Height affects how smoothly you can height paint. If it’s set really high, then the side of a small hill will look more squared-off. Can think of it, sort of, as more max ht is more rounding off. I’ve never seen a noticeable difference, but some doc somewhere suggests setting Height as low as possible (to the most you will ever need) in order to get smoother hills.