Refers to boards that are milled to press flush against one another completely. Nearly all older hardwood flooring was milled this way which may be a consideration if you want to match the look of other rooms in your home.
Micro Bevel (2 sides)
Refers to a fine milled edge that runs down the long sides of each board. The ends of the boards do not have this edge and fit together just as Flat edge boards do.
Micro Bevel (4 sides)
Refers to a fine milled edge on all 4 sides of each board.