The Path Marker is an item in BuildCraft that creates lines for a Builder to follow to build multiple copies of a structure. Unlike Land Marks, Path Markers do not need to be on the same axis. The farthest distance Path Markers can be placed from each other to activate is 64x64x64 blocks from the Path Marker.
To have the Builder build multiple copies of an item, say a path or a bunch of fountains in a row, use Path Markers before placing the Builder. Place Path Markers down and right-click them to activate them. They will connect to the nearest Path Marker and specify a red line by which the Builder will follow. This line can be diagonal and does not have to be on the same level as others. Then place the Builder next to the first Path Marker and power it. The Builder will then build structures following the lines specified. The Path Markers will pop out of the ground and can be reused later.
Powering a Path Marker with a Redstone Torch will create a blue beam that helps line up Path Markers on the same axis, if needed. This beam will pass through terrain for up to 63 blocks.