When naming a segment, you typically use the notation that highlights the end points of the segment. For example, if you have a line segment with endpoints A and B, you would denote it as AB or segment AB. This indicates that it is specifically a segment and not just any line.
In contrast, when naming a line, you generally use a single lowercase letter or two points to indicate the line. For example, a line that passes through points A and B can simply be referred to as line l or line AB. The key difference is that the designation for a segment focuses on the specific part between the points, while the designation for a line implies an infinite expanse in both directions.