A scratch golfer is someone who can play to a Course Handicap of zero on any and all rated golf courses. They are expected to play the course in the number of strokes equal to the course's par on average. The difficulty of a course is usually gauged using the Course Rating rather than Slope Rating which indicates the expected score for a scratch golfer playing under standard conditions.
A bogey golfer is a player with a handicap of around 18 for a male and 22 for a female. The Slope Rating of a course is used to measure the relative difficulty for bogey golfers compared to scratch golfers. A higher Slope Rating indicates a greater likelihood that a bogey golfer's score will rise more quickly than a scratch golfer's score on a more difficult course.
For scratch golfers, the Course Rating is a better indicator of the course's difficulty.
For bogey golfers and others with handicaps, the Slope Rating is more relevant. A higher Slope Rating means that the course will be relatively harder for non-scratch players compared to scratch players.
While the Course Rating directly communicates the difficulty for scratch players, the Slope Rating gives insight into how much more challenging a course might be for the average player compared to those scratch players.