SF.12: Prefer the quoted form of #include for files relative to the…

CppCoreGuidelines.md

@@ -18742,7 +18742,7 @@ Source file rule summary:
 * [SF.9: Avoid cyclic dependencies among source files](#Rs-cycles)
 * [SF.10: Avoid dependencies on implicitly `#include`d names](#Rs-implicit)
 * [SF.11: Header files should be self-contained](#Rs-contained)
-* [SF.12: Prefer the angle bracket form of `#include` where you can and the quoted form everywhere else](#Rs-incform)
+* [SF.12: Prefer the quoted form of `#include` for files relative to the including file and the angle bracket form everywhere else](#Rs-incform)
 
 * [SF.20: Use `namespace`s to express logical structure](#Rs-namespace)
 * [SF.21: Don't use an unnamed (anonymous) namespace in a header](#Rs-unnamed)
@@ -19181,27 +19181,27 @@ A header should include all its dependencies. Be careful about using relative pa
 
 A test should verify that the header file itself compiles or that a cpp file which only includes the header file compiles.
 
-### <a name="Rs-incform"></a>SF.12: Prefer the angle bracket form of `#include` where you can and the quoted form everywhere else
+### <a name="Rs-incform"></a>SF.12: Prefer the quoted form of `#include` for files relative to the including file and the angle bracket form everywhere else
 
 ##### Reason
 
 The [standard](http://eel.is/c++draft/cpp.include) provides flexibility for compilers to implement
 the two forms of `#include` selected using the angle (`<>`) or quoted (`""`) syntax. Vendors take
 advantage of this and use different search algorithms and methods for specifying the include path.
 
-Nevertheless, the guidance is to use the angle form when possible. This supports the fact that the
-standard library headers must be included this way, is more likely to create portable code, and enables
-the quoted form for other uses. For example being clear about the locality of the header relative
-to files that includes it or in scenarios where the different search algorithm is required.
+Nevertheless, the guidance is to use the quoted form for including files that exist at a relative path to the file containing the `#include` statement (from within the same component or project) and to use the angle bracket form everywhere else, where possible. This encourages being clear about the locality of the file relative to files that include it, or scenarios where the different search algorithm is required. It makes it easy to understand at a glance whether a header is being included from a local relative file versus a standard library header or a header from the alternate search path (e.g. a header from another library or a common set of includes).
 
 ##### Example
 
-    #include <string>       // From the standard library, required form
-    #include "helpers.h"    // A project specific file, use "" form
+    // foo.cpp:
+    #include <string>                // From the standard library, requires the <> form
+    #include <some_library/common.h> // A file that is not locally relative, included from another library; use the <> form
+    #include "foo.h"                 // A file locally relative to foo.cpp in the same project, use the "" form
+    #include "foo_utils/utils.h"     // A file locally relative to foo.cpp in the same project, use the "" form
 
 ##### Note
 
-Failing to follow this results in difficult to diagnose errors due to picking up the wrong file by incorrectly specifying the scope when it is included.
+Failing to follow this results in difficult to diagnose errors due to picking up the wrong file by incorrectly specifying the scope when it is included. For example, in a typical case where the `#include ""` search algorithm may search for a file existing at a local relative path first, then using this form to refer to a file that is not locally relative could mean that if a file ever comes into existence at the local relative path (e.g. the including file is moved to a new location), it will now be found ahead of the previous include file and the set of includes will have been changed in an unexpected way.
 
 Library creators should put their headers in a folder and have clients include those files using the relative path `#include <some_library/common.h>`