Data Sources can have In and Out streams. Each part can have 0, 1 or more of these.
In Streams
Most parts expect certain In-streams, usually Default. Part that specifically expect another stream will show that as well, like the Fallback in on certain filters, which contain the data to be forwarded if the filter returns nothing. See the ValueFilter example:
Other parts may allow a variable about of in-streams with names that you can pick.
Out Streams
The same thing applies to Out-streams. Many DataSources only have a single out:
Others have a dynamic amount of Out-streams like the App DataSource. Connect these Out-streams to In-streams how you want it. DataSources with a dynamic amount of Out-streams show it with a yellow 🔼 when you mouse over them.
More about Streams