Let's say that +X is pointing up. Imagine stepping onto an elevator with a scale to measure force (weight) and a 1 kg mass. When you put the mass on the scale the force shows as 9.8 N when the elevator is at rest.Â Now push a button to go up.Â The elevator accelerates in the +X direction and the force on the scale increases until the elevator reaches a constant velocity. As the elevator decelerates as it approaches the top floor, the force on the scale decreases until the elevator comes to a stop.

You could simulate that in Ansys by attaching a 1 kg mass below a spring along the X axis (and translation joint) and request results for the force in the spring. Apply a 9.8 m/s^2 acceleration in the +X direction, the force will be 9.8 N and the spring will stretch. That is like a stationary elevator. Change the acceleration to 15 m/s^2 and the force will be 15 N and the spring will stretch some more. In other words, the upward acceleration (+X) pulls the mass down (-X). That is like the elevator accelerating upward. Change the acceleration to 5 m/s^2 and the force will be 5 N. That is like the elevator decelerating as to approaches the top floor. This makes intuitive sense to me.

Ansys chose to define Standard Earth Gravity directions by the direction the mass will be pulled, rather than the direction of acceleration because that makes intuitive sense to many people.