What does "Expected Score" mean?

Expected score, Rules of Handicapping

A new definition in the Rules of Handicapping effective 1/16/24, "expected score" is the score a player is expected to achieve over a specified number of holes on a course of standard difficulty. It is calculated using the player's Handicap Index and attributes a numerical value against any hole or holes not played during a round. 

The expected score is based on the average Score Differential of a player with a given Handicap Index and a normal distribution of scores - so it is not specific to each player. An expected score can be thought of as a neutral value, meaning that a good 9-hole round (relative to the player's ability) will result in a good 18-hole Score Differential. An average 9-hole round will result in an average 18-hole score Differential, etc. 

There are three places in the Rules of Handicapping where expected score is used: 1) when the player posts all nine holes on a rated nine hole tee, the score will be "scaled up" to an 18 hole score differential, 2) when the player plays between 10-17 holes, the score will be "scaled up" to an 18 hole score differential, and 3) in the formula of the PCC (Playing Conditions Calculation) adjustments procedure.