diff --git a/intro.md b/intro.md
index b99e045..db89654 100644
--- a/intro.md
+++ b/intro.md
@@ -14,6 +14,8 @@ kernelspec:
 
 ASHRAE standard 223P is a proposed standard that formally defines knowledge concepts for representing building system information such as the connections between mechanical equipment, spaces within a building, and their measurement and control points. It is a metadata schema and semantic ontology that defines the modeling constructs and rules needed to construct semantically interoperable, machine-readable semantic models that provide software applications the ability to determine essential information about the meaning and context of building data to support the deployment of various advanced features such as advanced building controls, fault detection and diagnostics, and automated commissioning.
 
+Most models are available in two variants: "original" and "compiled" where in the latter, SHACL Rules defined by the 223P ontology are applied to extend RDF instance data.
+
 ````{margin}
 ```{note}
 The 223P standard is not yet finalized and is still under development.