The main purpose of these rails is to help you convert a standard clamp from different manufacturers into a rail/clamp combination used as nodal slide for your gear. You can mount any size clamp onto these rails and are not limited to a 1.00 or 1.50 inch clamp.
Some camera L brackets are designed with cuts on the sides to let you access a connection on the left side. If this is the case, a 1.50 inch clamp or nodal slide with integrated clamp will not work because there is simply no room to engage the clamp jaws. Heavier equipment also requires a wider clamp.
If you own a clamp from any manufacturer you can mount it on these rails. The packages of individual rails includes 2 dowel pins that prevent the clamp from twisting. If you own a clamp with a groove it will help placing the pin(s) into the rail hole and aligning it with the groove on the clamp. My own Hejnar Photo clamps incorporate extra holes for aligning the clamp on the rail and preventing it from twisting.
Some clamps you will have to mount without any pins, since they do not have any opening where the anti twisting pins can be inserted. In that situation you can use an index stop bar (sold separately) to prevent the clamp from twisting. As you can see on the pictures, the clamp can be placed on the rail in orientation where the clamp knob is on its back or on its side.
If you want the clamp to go higher so that the end of the rail will not be visible in your viewfinder you can use a 1/4 or 3/8 spacer block (sold separately.)