It's a sign the ball *ended* in his hands.
Scenario: Baker brings up the ball against pressure, crosses the midline and passes it to Johnson at the top of the arc, who fakes a handoff to Harper then hands it off to Mulcahy, who penetrates to draw a double team and whips the ball to the corner for Young, who loses it out of bounds on a drive to rim. The only person who was "used" in that play was Young, according to "usage rate".... and he may have had it in his hands for just 1-2 seconds of the shot clock.
Scenario: Mulcahy brings up the ball, and passes it to Harper on the wing. Harper backs in toward the lane but draws a double, kicking it back out to Mulcahy, who gives it over to Baker. Baker penetrates to the elbow drawing the defense, and kicks to Mathis who bricks a three on a catch-and-shoot. Only Mathis was "used" on that play, and had the ball in his hands less than a second.
Scenario: Young brings up the ball fighting against pressure, probing the defense, not finding an opening, backing back to the line, then again aggressively penetrating to the hoop and drawing the shot blocker..... only to give a last second pass underneath to Johnson, who jams it home. Only person who got "used" on that was Johnson, even though Young had the ball in his hands nearly the entire possession.
I just think "usage" is not the right word for that. It's not about who has the ball in their hands, but who ends with the ball on each possession - either with a make, a miss, or a turnover.