Devicemotion.useracceleration Acceleration Direction

As shown in the iOS coordinate system, if the phone is subjected to 1 forces along the X-positive direction (assuming the force generates 1m/s^2 acceleration), what is the value of devicemotion.useracceleration?

The answer is contrary to common sense: Devicemotion.useracceleration = ( -1/9.8, 0, 0)

That is, the accelerometer returns a value that is opposite the direction of the force.

As there is no obvious information on the Internet, it is hereby recorded.

Note: In iOS, the unit of the accelerometer return value is 9.8m/s^2

Devicemotion.useracceleration Acceleration Direction