It is a (very) common task to want to call the server for more information, or deliver information for processing. To do this within an action graph you use Invoke Model.
The value of the element is the name of the model (within the User's space or a shared model) to be invoked.
To pass parameters to the model being invoked, attach an Parameter element to the Invoke, in which case the value of that parameter will be supplied on the call.
If you wish a specific value to be passed for a Parameter value, perhaps to override the value in memory, or perhaps the name you wish to use is not currently a parameter, then attached a Value element to the Parameter element. In which case the value of the Value element will be used and passed to the called model as if it was the Parameter's value.