Once a project has been opened, the existing project items that are Fortran source code will be scanned, recursively, for Fortran include files (specified by the INCLUDE statement). These files, if found, will be added as a sub-node to the including (parent) project item. The added sub-node has its Build Action property set to "None" since these files are not a direct input to the compiler.
If an INCLUDE statement is added to or removed from a source file during editing, a sub-node will be created or removed when the file is saved to disk.
To open an include file for viewing or editing, double-click on the sub-node in the Solution Explorer, or right-click on the file name in an INCLUDE statement and select "Go To Include" from the context menu.