Simscape uses a generalized version of Kirchhoff's Circuit Laws. In the diagram you showed, those 3 nodes are actually the same node because they are directly connected to each other with no components in between. In other words, there are 4 block ports sharing that node.
What this tells you about the node is:
- The "voltage" is the same at all the block ports connected to that node.
- The total "current" entering/leaving that node must equal to 0.
For each different domain, the "voltage" or "current" equivalents are different, and are referred to as Across and Through variables.
In the Pneumatic domain, to follow your screenshot:
- Across variables: Pressure and Temperature
- Through variables: Mass flow and heat flow
- Sebastian
