The name of the NT user account under which to start this service. Leave this field blank to use the LocalSystem user account. This field is resolved at runtime before it is used.
The MSDN Win32 API documentation for the CreateService API describes this setting as:
If the service type is SERVICE_WIN32_OWN_PROCESS, use an account name in the form DomainName\UserName. The service process will be logged on as this user. If the account belongs to the built-in domain, you can specify .\UserName.
See the CreateService Win32 API help documentation for more details. This value is passed as the lpServiceStartName parameter.