Grid
From Unofficial Stationeers Wiki
Stationeers uses a global gridsystem to align every placed structure.
Big Grid
The big grid is primary used by big structures like Frames, Wall, Windows and others.
Small Grid
The size of the small grid is a quarter of the big grid. Important is that it is offset by 0,5. That means that the center of a big grid is at the same time the center of a small grid.