Why do the rules differentiate between water hazards and lateral water hazards? With a "regular" water hazard (that is, one that is not lateral), one of the options available to a golfer who hits into the hazard is to drop a ball behind the hazard. But a lateral water hazard might run the full length of a hole, down one side or the other. Dropping behind such a hazard is impractical, perhaps even impossible.
So relieve from lateral water hazards includes another option for golfers, which is addressed in Rule 26.
The official definition of "lateral water hazard" is "a water hazard or that part of a water hazard so situated that it is not possible, or is deemed by the Committee to be impracticable, to drop a ball behind the water hazard in accordance with Rule 26-1b."
Lateral water hazards should be defined on a course by the use of red stakes or red lines.