What do x, y, and z denote in mavros topic /mavros/imu/data_raw

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hi, I am using mavros to read the signals from the APM2.5. However I am not sure what x, y, z denote.

For example, in angular_velocity, I think the signals should be pitch, roll, and yaw in rad. How do x, y, and z correspond to them. Also, in linear_acceleration, what is the coordinates for x, y, and z, and are they in meter? Many thanks.

$ rostopic type /mavros/imu/data_raw
sensor_msgs/Imu
$ rosmsg show sensor_msgs/Imu
std_msgs/Header header
  uint32 seq
  time stamp
  string frame_id
geometry_msgs/Quaternion orientation
  float64 x
  float64 y
  float64 z
  float64 w
float64[9] orientation_covariance
geometry_msgs/Vector3 angular_velocity
  float64 x
  float64 y
  float64 z
float64[9] angular_velocity_covariance
geometry_msgs/Vector3 linear_acceleration
  float64 x
  float64 y
  float64 z
float64[9] linear_acceleration_covariance