Standard Hiwin rail ("R" designation) has a through-hole for mounting. A counterbore is located on the top of the rail. Optional caps are available to cover the hole once the rail is installed to keep this area clear of debris. For most applications, this is the rail of choice. Please pay particular note to the "E" and "P" dimensions. On uncut factory rail, the "E" measure is the distance from the end of the rail to the center of the first mounting hole. This can be a critical point if you are replacing an existing rail section. The "P" dimension is the mounting hole center-to-center measure.