Can anybody explain what this is and how it works?

I'm having a bit of a problem at the moment with a maximum size worldspace.

Normally, I stick to 4096x4096 worlds and selecting the Skyrim 1:1 always works fine. It has 10000000 (10 million) beside it. So I figure that if 4096x4096 works with 10 million, a worldspace that's 16384x16384 should be a figure 16x bigger...160million. Nope, doesn't work.

Tried various other pre-sets...they don't work either. Every one of them goes to error -1 a couple of seconds after clicking generate.

Oscape loads and displays the heightmap without any problem.

Can anyone elaborate on this?