#223 update URI explanation

Author: atomczak

Documentation/UserManual/classification-facet.md

@@ -14,7 +14,7 @@ IFC models keep track of **Classification** names, dates, versions, and other da
 | ---------- | -------- | -------------------- | -------------------------------------------------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
 | **System** | ✔️       | ✔️                 | The name of the **Classification System**                            | The element must be classified with a reference that is part of a classification system with this name                                                                                                                                                                  |
 | **Value**  | ❌       | ✔️                 | The value of a **Refeference** code in the **Classification System** | The element must be classified with a **Reference** which has a code that matches this value. The value is typically a short code with a separating character that denotes the level of classification                                                                  |
-| **URI**    | ❌       | ❌                   | Uniform Resource Identifier of the class. The resource should include a name and a definition, and preferably comply with ISO 23386. | One source of valid URIs is [the bSDD](https://search.bsdd.buildingsmart.org/). An example URI for a "beam": [https://identifier.buildingsmart.org/uri/buildingsmart/ifc/4.3/class/IfcBeam](https://identifier.buildingsmart.org/uri/buildingsmart/ifc/4.3/class/IfcBeam). |
+| **URI**    | ❌       | ❌                   | Uniform Resource Identifier of the class. | Used to reference a standardized definition of a class, to ensure consistentcy of interpretation. The target resource should include a name and definition, and preferably comply with the ISO 23386. This is an optional attribute that is not subject to IDS checking - the IFC model does not need to have the same or any URI. One source of valid URIs is [the bSDD](https://search.bsdd.buildingsmart.org/), and an example URI is that of a "Beam": [https://identifier.buildingsmart.org/uri/buildingsmart/ifc/4.3/class/IfcBeam](https://identifier.buildingsmart.org/uri/buildingsmart/ifc/4.3/class/IfcBeam). |
 
 If no parameters are specified, then it means that any **Classification** should be present, regardless of **System** name or **Reference** code.