You can specify particular user account credentials in a central location to be used for all agents, except for cloud agents which always use a dedicated user. This feature allows agents to be launched under a specific user account rather than the default SYSTEM account. For technologies such as browser-driven web, it is useful and recommended to run the virtual users in the context of a user account instead of the Windows built-in SYSTEM account. When you enable this setting, make sure that the specified user account is available on all the agent machines that are used for the load test.
By default agents are assigned to the SYSTEM account, but you also have the option to specify a dedicated user account to an agent through the System Configuration Manager (see Using a Particular User Account for a single Agent). Using this method to assign a specific user account can be cumbersome if you have a large amount of agents, because you have to specify user account credentials separately for each agent.