What is, and how do I control, the event that triggers the agent execution in rlACAgentOptions when SampleTime = -1

I'm trying to trigger the "step" function whenever the external "Observation" data changes, (as opposed to on a timed basis), so I set the SampleTime parameter to -1 as per the documentation below.

1. What events trigger the agent execution?
2. How do I control those events?

rlACAgentOptions

Sample time of agent, specified as a positive scalar or as -1. Setting this parameter to -1 allows for event-based simulations.

Within a MATLAB® environment, the agent is executed every time the environment advances. In this case, SampleTime is the time interval between consecutive elements in the output experience returned by sim or train. If SampleTime is -1, the time interval between consecutive elements in the returned output experience reflects the timing of the event that triggers the agent execution.

Thank you in anticipation

Gary Marshall