What does the bit depth of 3D mean?

3D or 5D refers to the groove length/diameter of the drill bit

For example, 3D, the drill diameter is 20MM, the effective drilling depth is 60MM, but the actual standard drill bit length should be about 80mm. Because the chip space should be considered!!

6mm x 22mm 90 Degree 3D Wood Making Router CNC Engraving V Groove Bits Drill Tool Woodworking Carve

Use for Large Routing Jobs Such as Kitchen Wood Cabinet Doors,
Cabinet Drawers, Intricate Detailing on Furniture and More.