Add discussion of processing frameworks being responsible for all def…

…ined attributes and tags (#2417)

and refer to it for sortorder and protect attributes

Closes #2395

registry.adoc

@@ -203,6 +203,40 @@ There are a variety of output generators included:
   * `extensionStubSource.py` - generate a simple C extension loader.
 
 
+[[implementations]]
+= Alternate Registry Processing Frameworks
+
+We encourage people needing to process registry XML to use the Python
+generator framework.
+It has been successfully used by many other projects, such as OpenXR and
+many of its software components
+
+Be aware that the schema is not static.
+Over time, we have introduced a number of non-breaking changes in the form
+of additional XML tags and attributes.
+On rare occasion we have made a breaking change, such as replacing the
+attr:requires syntax and attr:requiresCore attributes for specifying
+extension and version dependencies with the attr:depends attribute.
+
+While such changes are supported in the Python scripts when they are made,
+downstreams using other XML processing code are responsible for tracking and
+implementing corresponding changes.
+
+
+[[implementations-rare]]
+== Rarely Used Tags and Attributes
+
+If you do implement your own XML processing framework, you are responsible
+for complying with the interpretation of the schema and its contents, and
+producing results consistent with the Python framework.
+
+Choosing to not implement processing of certain tags or attributes may
+produce incorrect outputs.
+If you are doing this because the tags or attributes are not currently used
+in the Vulkan XML, you may want to flag a warning or error when they are
+encountered.
+
+
 [[schema]]
 = Vulkan Registry Schema
 
@@ -1355,15 +1389,24 @@ feature.
     A decimal number which specifies an order relative to other tag:feature
     tags when calling output generators.
     Defaults to `0`.
-    Rarely used, for when ordering by attr:name is insufficient.
+    Used when ordering by attr:name is insufficient.
   * attr:protect - optional.
     An additional preprocessor token used to protect a feature definition.
     Usually another feature or extension attr:name.
-    Rarely used, for odd circumstances where the definition of a feature or
-    extension requires another to be defined first.
+    Used when the definition of a feature or extension requires another to
+    be defined first.
   * attr:comment - optional.
     Arbitrary string (unused).
 
+[NOTE]
+====
+The attr:sortorder and attr:protect attributes may not be used
+in the XML at times.
+If writing an <<implementations, alternate registry processing framework>>,
+refer to the notes on <<implementations-rare, rarely used tags and
+attributes>>.
+====
+
 [NOTE]
 ====
 The attr:name attribute used for Vulkan core versions, such as
@@ -1459,7 +1502,7 @@ implemented against.
     A decimal number which specifies an order relative to other
     tag:extension tags when calling output generators.
     Defaults to `0`.
-    Rarely used, for when ordering by attr:number is insufficient.
+    Used when ordering by attr:number is insufficient.
   * attr:author - optional.
     The author name, such as a full company name.
     If not present, this can be taken from the corresponding tag:tag
@@ -1520,8 +1563,8 @@ logical AND, and must be replaced by `+` in the `depends` attribute.
     An additional preprocessor token used to protect an extension
     definition.
     Usually another feature or extension attr:name.
-    Rarely used, for odd circumstances where the definition of an extension
-    requires another extension or a header file to be defined first.
+    Used when the definition of a feature or extension requires another to
+    be defined first.
   * attr:platform - optional.
     Indicates that the extension is specific to the platform identified by
     the attribute value, and should be emitted conditional on that platform
@@ -3213,6 +3256,9 @@ Changes to the `.xml` files and Python scripts are logged in GitHub history.
 [[changelog]]
 = Change Log
 
+  * Added notes on <<implementations, alternate registry processing
+    frameworks>> and referred to them for rarely used attributes in the
+    <<tag-feature, tag:feature>> tag (public issue 2395).
   * 2024-07-18 - Remove out of date guidance that builtin and external types
     should not be wrapped in tag:type tags, replaced with actual examples in
     the <<tag-type, tag:type>> section (public issue 2394).