What is role of S-MSCKF VIO

[nvhungv2k]

In the case of gazebo simulation, I ran it completely as tutorial video in Wiki. One thing I don't understand that: In default, it don't use S-MSCKF VIO (because onboard_sensing = false in full_sim.launch). So which source the planners get the location from to create path and trajectory? I make it used S-MSCKF VIO by setting onboard_sensing = true in full_sim.launch. S-MSCKF VIO published to /quadrotor/ukf_odom topic, BUT no one used it (that is, no subscriber):

And I don't see any differences between using or not S-MSCKF VIO in gazebo simulation. So in this case which role does S-MSCKF VIO play? I supposed that:

1. It only is used in real robot. But with simulation, odometry is got from gazebo directly
2. It is as backbone for any SLAM (ex: SLOAM,...) to correct the drift.

Are my these insights right? PLS show me more explaination. Thanks so much!

[XuRobotics]

See https://github.com/KumarRobotics/kr_autonomous_flight/issues/194#issue-2369823327