HERE
400X400X10 AND 400X400X10
There aren't any major differences, so it's okay.
There may be issues with procedural floor structures.
More details here.
If you place the floors in the same location, they must overlap.
use a good name
You do this to ensure they appear in order as you go through them.
right click => create child blueprint class
Now open the created blueprint.
go back to "DT_BuildParts" and use your floor details.
Icon details:
Dimensions: 512x512
No Background/ transparent
You just have to refer to the specific category.