The cells in a system communicate status with each other. For example, when you change a mesh in the Mesh cell, the Setup cell will report that its software (CFX-Pre) requires a refresh to re-read the "upstream" data.

Similarly, the same type of data flow occurs between systems. For example, if you have an FSI: Fluid Flow system (as shown in the image below), data and status from the Fluid Flow (CFX) Geometry cell will flow both to the Fluid Flow (CFX) Mesh cell and the Static Structural Geometry cell. Data flow between systems is shown by interconnecting lines: