@@ -99,6 +99,7 @@ parts:
   - caption: "Documentation 📚"
     chapters:
       - file: docs/intro
+      - file: docs/xref-reference
         sections:
         - file: docs/user-documentation
         - file: docs/developer-documentation
 
@@ -0,0 +1,59 @@
+(xref-reference)=
+# MyST `xref` reference
+
+This page provides serves some reference information about JupyterBook 2 / MyST xref usage in our documentation ecosystem.
+Specifically, this page includes:
+ * our approach for avoiding circularity of xrefs (e.g., if book 1 and book 2 xref each other, and both have updates, the build order can potentially impact the freshness of the content that shows up in each)
+ * canonical labels and links used for our most common `xrefs`.
+
+# `xref` "levels"
+
+A resource at a given "level" (listed below) should only xref to resources at numerically lower levels.
+For example, Level 2 resources should only xref to resources in Levels 0 or 1.
+
+This information is used when documentation is rebuilt.
+Builds are performed in numeric order of the levels - so first all Level 0 resources build, then all Level 1 resources, and so on.
+This is particularly important for xref-embedded content: it ensures that most recent version of content is embedded in a Level N resource from a Level N-1 resource when the Level N resource is built.
+An example of this is the glossary, which is [embedded in *Using QIIME 2*](https://github.com/rachis-org/using-qiime2/blob/main/book/back-matter/glossary.md?plain=1) and [in amplicon-docs](https://github.com/qiime2/amplicon-docs/blob/main/docs/back-matter/glossary.md?plain=1) through an xref to content in [*Rachis News*](https://github.com/rachis-org/news.rachis.org/blob/main/blog/glossary.md?plain=1).
+
+Build order within a level is not important.
+
+## Level 0: unversioned resources
+
+| Name | Description | Target | URL |
+| ---- | ----------- | ------ | --- |
+| `rachis` news | glossary source (and potentially other broadly referenced source content in the future) | `rachis-news-target` | <https://news.rachis.org/en/latest/> |
+| library | plugin references | `rachis-library-target` | <https://library.qiime2.org> |
+
+## Level 1: framework references
+
+| Name | Description | Target | URL |
+| ---- | ----------- | ------ | --- |
+| Developing with QIIME 2 | developer manual | `developing-with-rachis-target` | <https://develop.qiime2.org/en/latest/> |
+| Using QIIME 2 | rachis user manual | `using-rachis-target` | <https://use.qiime2.org/en/latest/> |
+
+## Level 2: data-set focused tutorials
+
+| Name | Description | Target | URL |
+| ---- | ----------- | ------ | --- |
+| Gut-to-soil tutorial | user-focused QIIME 2 tutorial | `gut-to-soil-target` | <https://gut-to-soil-tutorial.readthedocs.io/en/latest/> |
+| Moving pictures tutorial | user-focused QIIME 2 tutorial | `moving-pictures-target` | <https://moving-pictures-tutorial.readthedocs.io/en/latest/> |
+
+## Level 3: Examples of distribution and plugin documentation
+
+At this moment, this is the highest level of xref, so nothing xrefs into these.
+For that reason, canonical targets aren't yet defined.
+
+| Name | Description | Target | URL |
+| ---- | ----------- | ------ | --- |
+| amplicon-docs | QIIME 2 user manual | *no target defined* | <https://amplicon-docs.qiime2.org/en/latest/> |
+| moshpit docs | MOSHPIT user manual | *no target defined* | <https://moshpit.qiime2.org/en/latest/> |
+| genome-sampler docs | `genome-sampler` documentation | *no target defined* | <https://genome-sampler.readthedocs.io/en/latest/> |
+| q2-fmt docs | `q2-fmt` documentation | *no target defined* | <https://q2-fmt.readthedocs.io/en/latest/> |
+
+
+# Miscellaneous notes
+
+ * Remember that anchors can be found in the `myst.xref.json` at the top-level of the target's URL, for example: <https://news.rachis.org/en/latest/myst.xref.json>.
+ * [MyST xref docs](https://mystmd.org/guide/website-metadata#myst-xref-json)
+ * [MyST cross-referencing docs](https://mystmd.org/guide/cross-references)