The Strength of Field for each tours event is determined by using the World Rating and Home Tour Rating.
The World Rating is based on the number of Top-200 World Ranked players competing in the event and a value is allocated to the position within the Top 200.
The Home Tour Rating is based on the number of Top-30 Ranked players using each tours end of year final ranking with a value allocated to the position within the Top 30 Ranked players.
The World and Home Rating are added together to produce the Strength of Field rating. The number of points** allocated to an event is then determined by the "Points Breakdown" and which band the total Strength of Field rating falls into.
A more detailed explanation can be found under "How the Ranking Works"