Clarify usage of AbstractEntity as datatype

documentation/modules/ROOT/pages/modeling-guidelines.adoc

@@ -566,10 +566,8 @@ include::example$extending-entity-declaration-inheritance.ttl[tags=extending-ent
 [[declaring-abstract-entities]]
 ==== Declaring Abstract Entities
 
-In contrast to Entities, Abstract Entities cannot directly be used as data types. The main use case
-for Abstract Entities is to share Properties across multiple Entities. This can be achieved through
-inheritance: Abstract Entities can extend other Entities and Abstract Entities and can themselves be
-extended in a manner not dissimilar to how abstract classes are used in many programming languages.
+In contrast to Entities, Abstract Entities cannot directly be instantiated. A Characteristic class may have a type `samm:entity` or `samm:AbstractEntity`. But for a Characteristic instance, it cannot have a type `samm:AbstractEntity`. If there is an instance created from a Characteristic that uses an Abstract Entity as its type, this Characteristic instance's dataType must be a sub-Entity of the Abstract Entity.
+The main use case for Abstract Entities is to share Properties across multiple Entities. This can be achieved through inheritance: Abstract Entities can extend other Entities and Abstract Entities and can themselves be extended in a manner not dissimilar to how abstract classes are used in many programming languages.
 
 In the following example, both `SteeringWheel` and `Seat` _extend_ the Abstract Entity
 `VehicleComponent`. Both inherit the `typeNumber` and the `manufacturer` Properties:

esmf-semantic-aspect-meta-model/src/main/resources/samm/meta-model/2.2.0/aspect-meta-model-shapes.ttl

@@ -468,6 +468,8 @@ samm:CharacteristicShape
             filter ( not exists { $this rdf:type/rdfs:subClassOf* samm-c:Collection } )
             # exclude 'Trait' Characteristic since it refers to the datatype via its baseCharacteristic
             filter ( not exists { $this rdf:type/rdfs:subClassOf* samm-c:Trait } )
+            # exclude 'AbstractEntity' Characteristic since it is not allowed for Characteristic instance
+            filter ( not exists { $this rdf:type/rdfs:subClassOf* samm:AbstractEntity } )
             # find all characteristic instances which do not define a data type
             filter ( not exists { $this samm:dataType [] } ) .
             filter ( not exists { $this rdf:type/rdfs:subClassOf*/samm:dataType [] } )