The LOOKUP function is used to return the value of a specified field in a different row, which can be defined relative to the current row. In this context, it can be used to include a number with the position of each state within each region by looking up the SUM([Quantity]) in each respective position. Reference: The information is inferred from general Tableau knowledge as well as the following sources:
Tableau Functions (Alphabetical)
LOOKUP
Tableau Certified Data Analyst Exam Guide