Question: What does the □ mean -> skipped

[georgbachmann]

Quick question. In my output I see something like that:

[4d 0h 45m 13.147999999986496s]  84.3499% 216.89m/257.14m @  275/s | ✓ 216.89m □ 100.78m | 1d 16h 40m 8s left

So there are 100.78m tiles skipped. But what does that mean? Were there error generating those (if so... any change I can see what those are)? Is it save to use the resulting tileset? Or will there be missing things? Or is this just something like "There is no data here... e.g. sea"?!?

Couldn't find a README for that...