diff --git a/mkdocs.yml b/mkdocs.yml
index 1a24697360..04c07297ac 100644
--- a/mkdocs.yml
+++ b/mkdocs.yml
@@ -26,6 +26,7 @@ nav:
       - Glossary: glossary.md
       - BIDS Extension Proposals: extensions.md
       - Appendix:
+        - Schema: appendices/schema.md
         - Contributors: appendices/contributors.md
         - Licenses: appendices/licenses.md
         - Entity table: appendices/entity-table.md
diff --git a/src/appendices/schema.md b/src/appendices/schema.md
new file mode 100644
index 0000000000..1616a6d5c9
--- /dev/null
+++ b/src/appendices/schema.md
@@ -0,0 +1,19 @@
+# BIDS schema
+
+The BIDS schema is a machine readable representation of the BIDS standard,
+written in a custom YAML format.
+The goal of the schema is to provide a single source for rendering the specification
+and validating BIDS datasets, reducing the scope for inconsistencies.
+Third party tools may also use the schema to write code that will adapt to additions
+to the BIDS standard.
+
+The BIDS schema is available in two machine readable formats:
+
+-   as a set of [YAML](https://en.wikipedia.org/wiki/YAML) files in the [BIDS specification repository](https://github.com/bids-standard/bids-specification/src/schema)
+-   as a [single dereferenced json file](https://bids-specification.readthedocs.io/en/stable/schema.json)
+
+A didactic walkthrough of the schema can be found in the [BEP Guide](https://bids-extensions.readthedocs.io/en/latest/schema/),
+and a complete description is available in the [`bidsschematools` documentation](https://bidsschematools.readthedocs.io/en/latest/).
+[`bidsschematools`](https://pypi.org/project/bidsschematools/) is a Python package sourced
+from the specification repository, and includes the necessary code to render the specification
+and filename validation.
diff --git a/src/schema/objects/metadata.yaml b/src/schema/objects/metadata.yaml
index ea6e2fb5eb..0e1fe96916 100644
--- a/src/schema/objects/metadata.yaml
+++ b/src/schema/objects/metadata.yaml
@@ -500,6 +500,8 @@ ContrastBolusIngredient:
     - CARBON DIOXIDE
     - BARIUM
     - XENON
+    - UNKNOWN
+    - NONE
 DCOffsetCorrection:
   name: DCOffsetCorrection
   display_name: DC Offset Correction