When designing a subgraph, you might not know how an I/O port will connect in
the final system. It is probable that a subgraph output will link to an output_plio or output_gmio port.
To overcome this issue, you can define logical I/O ports such as
port<input>, port<inout>, and port<output> graph objects. This ensures that you can specify
the inputs and outputs of the subgraph. It also enables the end-user to determine
how those ports will physically connect in the system. These objects can establish
links between kernels within a graph and across levels of hierarchy in your
specifications that include platforms, graphs, and
subgraphs.