Each of these methods has advantages and disadvantages, discussed below.
Copying and pasting network elements
This is a simple method for joining scenarios into a larger one. Everything from one scenario can be copied and then pasted into another scenario within the same project (with the caveats listed in Copying Network Elements). Once you have pasted the network components, you can work with those components as you would for any scenario. A disadvantage of this method is that the copied scenario and the original are not linked in any way. Also, importing scenarios from other projects, thus enabling copying, is only possible in the full version of Source.
Scenario data sources
Using a scenario data source is a simple mechanism for connecting models, outputs from one or more scenarios, the donor scenarios, are used as inputs for another scenario - the acceptor scenario. First you run the donor scenario, then you select results to be used as data sources in the acceptor scenario, see Loading scenario data sources for more information.
The main advantage of this method is that it can improve performance on large models; you can split the model in to parts (eg. upstream and downstream) and run them sequentially. Furthermore, if you enable Reload on Run for a scenario data source, results from the latest run of the donor scenario will be used as input for the acceptor scenario, allowing you to pass results changes from the donor to the acceptor.
The disadvantage of this method is that information cannot be passed from the acceptor scenario to the donor scenario, so you cannot, for example, link ordering systems.
Scenario transfer node
The Scenario Transfer Node (STN) is a more complicated method for connecting models together, it is used to join scenarios within the same project with linked:
Ownership (only available in the licensed version of Source).
STNs can operate in both connected and disconnected modes. This allows you to develop different parts of a large model independently of each other as separate scenarios running in disconnected mode, and then you can run them together in connected mode. The disadvantage of this method is that when running in connected mode all scenarios that are connected to each other will be run, which may result in long run times. See Scenario Transfer Node for more information.