The Patch node is a patch within a patch (or a sub-patch). The patch node's inputs and outputs correspond to the input and output nodes within the sub-patch.
To enter the sub-patch, double-click on the patch node. To exit, select "Exit Sub-Patch" from the View menu.
A sub-patch can be shared between multiple patch nodes by instancing the patch node (accessible via the context menu).
A patch file may be imported as a sub-patch by selecting "Import Sub-Patch" from the File menu. This imports the file's data, rather than linking to the file.