angular_velocity_imu_in_imu

Eigen::Vector3d angular_velocity_imu_in_imu = Eigen::Vector3d::Zero()

Body angular velocity of the IMU frame in rad/s.

The full name of the body angular vector would be "angular velocity IMU to world in IMU", but as for all body angular velocities, "angular velocity Body to X in Body" is the same for all inertial frames X (that have zero velocity relative to the world frame). See for instance https://scaron.info/robotics/screw-theory.html#body-screws for the math.