Whereas normal tags in an XML document mark explicitly both the start and the end of an element and thus always enclose something, a milestone tag marks simply a point at which something changes, but does not enclose anything. Milestone number 42 on the road from London to Bath marks the point at which the forty-second section of that road begins.We can guess that if we keep going we will eventually reach milestone number 43, which will mark the point at which the forty-third section starts, and hence (since a stretch of road cannot be in two sections at once) the point at which the forty-second section finishes, but it is not explicitly marked.