From the Docs on CMAcceleration:
CMAcceleration
The type of a structure containing 3-axis acceleration values.
typedef struct {
double x;
double y;
double z;
} CMAcceleration;
X-axis acceleration in G's (gravitational force).
Y-axis acceleration in G's (gravitational force).
Z-axis acceleration in G's (gravitational force).
A G is a unit of gravitation force equal to that exerted by the earth’s gravitational field (9.81 m s−2).