Pathmarks are beacons used only in conjunction with Builders to specify a building path. Unlike Landmarks, Pathmarks do not need to be on one of the three axes to connect to one another. In this way, they can define paths at an angle, which means the build path can go up hills or down valleys, or make turns other than 90 degrees.
Like a Landmark, when a Pathmark is powered by Redstone, it emits a blue laser beam in all six cardinal directions, out to 64 blocks. Since Pathmarks don't need to be on the same axes, these beams are purely for measuring distances.
Once two or more pathmarks are set, the player simply right-clicks on one to start connecting them. The pathmark will then search for the next-nearest pathmark within a 129x129x129 cube centered on itself or, in other words, 64 blocks of any given direction. A pathmark can only be connected to two other Pathmarks, defining either a line or a loop, based on whether or not the player connects the last Pathmark to the first.