When designing a subgraph, it might not be feasible to know beforehand how an
I/O port will be connected 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.