Usage Rate estimates the percentage of team plays a player uses while on the floor — through field goal attempts, free throw trips, or turnovers. It measures how central a player is to the offensive structure.
Essential context metric — player efficiency stats only make sense alongside usage. A player shooting 58% TS% at 35% usage is doing something dramatically more difficult than a player shooting 58% TS% at 18% usage. Available historically.
Measures volume of possession use, not quality. A player who dominates the ball to the detriment of team flow can have a high USG% that flatters their counting stats while their RAPM metrics show neutral or negative impact.
USG% is included in APEX's Scoring pillar but as part of an interaction term rather than standalone. APEX v1.6 introduced a USG% × TS% efficiency-usage interaction penalty: high usage at low TS% is penalized; high usage at high TS% is rewarded proportionally. This is the primary analytical justification for APEX's de-ranking of 2016-17 Russell Westbrook (42.8% TS% at historically extreme usage) relative to models that don't account for this interaction. The Westbrook v1.6 flip is documented in APEX's version history as an intentional feature.