ARM CMSIS DSP PID Library - "state array" ?

typedef struct

  {

    float32_t A0;          /**< The derived gain, A0 = Kp + Ki + Kd . */

    float32_t A1;          /**< The derived gain, A1 = -Kp - 2Kd. */

    float32_t A2;          /**< The derived gain, A2 = Kd . */

    float32_t state[3];    /**< The state array of length 3. */

    float32_t Kp;               /**< The proportional gain. */

    float32_t Ki;               /**< The integral gain. */

    float32_t Kd;               /**< The derivative gain. */

  } arm_pid_instance_f32;