Definition

The following constraints apply to link objects:

1. Under any single <Aggregate Feature> instance or <Aggregate Geometry> instance, the link objects (if any) shall have non-identical field values or components, as appropriate. In the case of <Separating Plane Related Geometry> instances, this constraint applies to the link objects of the <Separating Plane Relations> components of the aggregate, as the aggregate itself does not have link objects.

2. Under any single topology hierarchy, whether a <Feature Topology Hierarchy> instance or <Geometry Topology Hierarchy> instance, the link objects shall have non-identical field values or components, as appropriate.

3. Under any single model instance, whether a <Feature Model Instance> instance or a <Geometry Model Instance> instance, the index field values of the <Model Instance Template Index> link objects shall be distinct.

4. Consider a <Feature Representation> instance F1 that associates to a <Feature Representation> instance F2 through the use of a <Base Association Data> link object S1 and separately through the use of a <Base Association Data> link object S2, where F1 and F2 may not be distinct.

   1. S1 and S2 shall be distinct link objects.

   2. If S1 and S2 are instances of the same concrete DRM class, the meaning field values of S1 and S2 shall be distinct and shall not conflict.

5. Consider a <Geometry Hierarchy> instance G1 that associates to a <Geometry Hierarchy> instance G2 through the use of a <Base Association Data> link object S1 and separately through the use of a <Base Association Data> link object S2, where G1 and G2 may not be distinct.

   1. S1 and S2 shall be distinct link objects.

   2. If S1 and S2 are instances of the same concrete DRM class, the meaning field values of S1 and S2 shall be distinct and shall not conflict.

6. Consider a <Feature Representation> instance F that associates to a <Geometry Hierarchy> instance G through the use of a <Base Association Data> link object S1 and separately through the use of a <Base Association Data> link object S2.

   1. S1 and S2 shall be distinct link objects.

   2. If S1 and S2 are instances of the same concrete DRM class, the meaning field values of S1 and S2 shall be distinct and shall not conflict.

7. Consider a <Geometry Hierarchy> instance G that associates to a <Feature Representation> instance F through the use of a <Base Association Data> link object S1 and separately through the use of a <Base Association Data> link object S2.

   1. S1 and S2 shall be distinct link objects.

   2. If S1 and S2 are instances of the same concrete DRM class, the meaning field values of S1 and S2 shall be distinct and shall not conflict.

Rationale

Discriminators shall be distinct in order to allow the user to discriminate between the branches they represent.

However, this does not apply to link objects that exist only to specify sides of a <Separating Plane> instance.

For model instances, the indexes into the <Variable> instances associated with an <Interface Template> instance shall be distinct because no <Variable> instance can be set to more than one value at a time.

Example

1. For a level of detail related aggregation, no two <Base LOD Data> link objects may be identical.

2. For a time related aggregation, two <Time Constraints Data> link objects may overlap, but not be identical.

3. Let L1 and L2 be distinct <Linear Feature> instances such that L1 is associated to L2 through a <Spatial Association Data> link object S1 with a meaning field value of SE_SPATASSMEANTYP_COMPLETELY_ABOVE. L1 may not also be associated to L2 through a <Spatial Association Data> link object S2 with a meaning field value of SE_SPATASSMEANTYP_COMPLETELY_BELOW, although L2 may be associated to L1 through such a <Spatial Association Data> link object.

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