Unit in UE4 ? basic vs BSP mesh dimension?

I’m kinda new to UE4. so may I ask what the number of scale stand for ? mm ? cm ? inch ? from what I’ve been doing, I think in UE4 . the scale is for metter.

and why in the same form but the scale of BSP Mesh 's only half of Basic Mesh . ( which makes the BSP twice bigger than basic Mesh ) . but I found out that the scale of basic Mesh is actually the right one in the reall world. so I always have to reduce the scale of BSP by half .

please help me. I appreciate it.

Scale doesn’t have a unit; it’s just a ratio. I think you understand that but mistyped. You meant to ask about Location’s units. The units have traditionally been called uu for “Unreal Unit” and were semi-arbitrary for performance reasons (powers of two). Nowadays, 1 UU = 1 CM.

Your BSP brush was set to 200x200x200 when you built that cube. That’s why you had to scale it to 0.5 to make it the same size as the Static Mesh. Look in your screenshot where it says Brush Settings. If you mouse over the Static Mesh (Basic Cube) in that rightside details pane, I bet a tooltip will pop up and tell you its dimensions are 100x100x100. There really isn’t much more to say about it besides that.

While using BSP still works for blocking things out / prototyping, many people will suggest you use static meshes everywhere. Google around for BSP vs static mesh.