When should you set use_sim_time to true

edit

use_sim_time should typically only be set to true if you are using a simulator/simulation environment, and your ROS nodes interface (ie: get their data, and send their data) with that simulator/simulation environment.

There are some situations in which more 'advanced' usage of it could be used to run partial systems on simulated time, but I would suggest to not do that if you're just starting out.

Thus, I am wondering if my understanding below is correct? I should only turn on 'use_sim_time' to be true for any code that is not related to any physical hardware, such as GPS, IMU, .. And for those nodes involving interacting with the physical component of the robot, should I set 'use_sim_time' to false?

Driver nodes (what you describe as "nodes involving interacting with the physical component of the robot" can typically not be used with simulated hardware (ie: a robot in Gazebo), so they are typically not started in such situations.

I've always considered Gazebo sensor and actuator plugins the equivalent of ROS drivers for real hw.

So, no, I don't believe what you write is correct.

I would suggest the following rule of thumb:

• using a simulation? Set use_sim_time to true for all nodes you start
• otherwise: set use_sim_time to false (or: don't set the parameter at all, as false is the default)