In some situations, the status of a Client could change in such a manner that the Controller would not be aware of the change (until it tried to send realizations to the Client). This could happen, for example, if a Client machine was turned off, or if it was rebooted prior to the beginning of the simulation.
The Update clients button provides a mechanism by which the Controller can recheck the status of the Clients prior to starting a simulation (in case some action needs to be taken, for example, if one of the Client machines has been disabled):
Pressing the Update clients button updates the status of all Clients.