Hierarchies, Relations, and Collections

The Three Fundamental Semantic Relations

Knowledge organization relies on three basic types of semantic relations between concepts:

1. IS-A (taxonomic/subsumption): A subtype relationship

   • Example: “Width IS-A dimensional property”

2. PART-OF (mereological/composition): A component relationship

   • Example: “Optical window PART-OF reactor”

3. HAS-A (attribution/possession): A property relationship

   • Example: “Substrate HAS-A width”

Understanding these distinctions is essential for correctly structuring vocabularies and avoiding semantic confusion.

Core Principles

Using skos:narrower / skos:broader for IS-A Relations Exclusively

The SKOS standard does not constrain the type of hierarchy expressed with the relations skos:broader and skos:narrower. While SKOS focuses on “IS-A” relations (car is a vehicle), expressing part-whole relations (tire is part of car) would also be possible. However, Voc4Cat is designed as a taxonomy that organizes concepts by subject. Therefore, the type of relations between concepts to create hierarchies are strictly “IS-A” relationships, and skos:broader and skos:narrower are only used to express IS-A (but not for PART-OF or HAS-A relations).

Thus, Voc4Cat focuses on categorizing things by what they are. Such subject hierarchies with IS-A based hierarchies correspond well with ontological modeling and reasoning as well as semantic search.

Correct usage:

:Width skos:broader :DimensionalProperty .
# "Width IS-A Dimensional Property" (taxonomic)

Anti-patterns:

:SubstrateWidth skos:broader :Substrate .
# Wrong: Confuses HAS-A ("Substrate HAS-A width") with IS-A

:OpticalWindow skos:broader :Reactor .
# Wrong: Confuses PART-OF ("Window PART-OF reactor") with IS-A

Single Location per Attribute Concept

Each attribute concept should appear once in the hierarchy at its most appropriate taxonomic level, based on IS-A relations.

Don’t create object-specific attribute concepts:

:Width a skos:Concept ;
    skos:broader :DimensionalProperty .

:SubstrateWidth a skos:Concept ;
    skos:broader :Width .
    
:FilmWidth a skos:Concept ;
    skos:broader :Width .

Problem: Unnecessary proliferation of concepts. While these technically follow IS-A logic, they create redundancy. The single concept Width can be applied to various objects (substrate, film, reactor) through data annotation metadata.

Create a generic attribute concept:

:Width a skos:Concept ;
    skos:broader :DimensionalProperty .
    # This single concept can be applied to substrates, films, reactors, etc.

Avoid Mixing Entity and Attribute Hierarchies

A more fundamental error is placing attribute concepts under entity concepts:

Never do this:

:Substrate a skos:Concept ;
    skos:broader :PhysicalEntity .
    
:SubstrateWidth a skos:Concept ;
    skos:broader :Substrate .
    # Wrong: "Width" is not a subtype of "Substrate"

Using skos:Collection for HAS-A and Contextual Relations

For other use cases, grouping concepts by topic is valuable. For such groupings, skos:Collection should be used. In Voc4Cat, skos:Collection may be used to create topic-wise lists or even for representing list-of-lists. In this topic-wise form of organization, PART-OF relations, HAS-A relations, or even looser relations (e.g., skos:related) dominate.

Use collections to express HAS-A relationships and group attributes by context:

• Object context (e.g., substrate attributes, reactor parameters)

  • Expresses: “Substrate HAS-A {width, thickness, material}”

• Measurement technique (e.g., XRF-measurable attributes)

  • Expresses: “XRF HAS-A {set of measurable attributes}”

• Application domain (e.g., photocatalysis parameters)

An example for such a collection in Voc4Cat is voc4cat:0007130, which collects the attribute terms for Scanning Electron Microscopy (SEM) measurements.

Expressing PART-OF Relations

PART-OF relationships between physical entities should not be modeled in Voc4Cat.

Example of what NOT to do:

:OpticalWindowThickness skos:broader :ReactorDimensions .
# Mixes PART-OF (optical window PART-OF reactor) with attribute hierarchy

PART-OF relations should be modeled in a separate data model or ontology which may reference concepts from Voc4Cat. Voc4Cat focuses exclusively on providing well-defined concepts that can be used as building blocks in such data models, but does not itself express compositional relationships between physical entities.

Context from Metadata, Not Vocabulary Structure

HAS-A and PART-OF relationships are expressed in data annotation metadata, not in the vocabulary hierarchy.

In data annotation:

• Attribute concept: URI reference to voc4cat:Width

• Measured object (HAS-A): Specified in metadata field (e.g., “substrate HAS-A width”)

• Object composition (PART-OF): Specified in metadata (e.g., “substrate PART-OF thin film”)

• Measurement device: Specified in metadata field

Benefits for Data Annotation

1. Semantic clarity: Each relation type handled appropriately

2. Unambiguous concept URIs: One URI per semantic concept following IS-A logic

3. Simple queries: “Find all width measurements” queries a single concept

4. Maintainable: No proliferation of concepts mixing relation types

5. Flexible: Collections express HAS-A relations without hierarchy pollution

6. Extensible: New objects or measurement techniques don’t require new attribute concepts

Voc4Cat Top-Concepts

The concepts included in Voc4Cat are structured under the following Top-Concepts:

	Compact URI	Top-Concept	Definition
1	voc4cat:0000180	Physical entity	Entity with a concrete and physical nature.
2	voc4cat:0000181	Non-temporal abstract entity	Any entity that cannot be located in space-time. E.g., mathematical entities, formal semantics elements, regions within dimensional spaces, ideas, models.
3	voc4cat:0000182	Temporal abstract entity	Anything that has a temporal dimension, whether it is an instantaneous point in time, a duration, or a sequence of events.
3.1	voc4cat:0000183	Events	An occurrence or happening, marked by a specific point in time. Events can be observed, recorded, and may have an impact on the state of the system or entities involved.
3.2	voc4cat:0000184	Actions	Temporal entities that have a duration and occur at specific points in time.
3.3	voc4cat:0000185	Processes	A series of temporal entities, like actions, events, changes, or functions that are not isolated but rather a connected sequence of activities. Processes often involve the transformation of inputs into outputs and can be conceptualized as workflows.
4	voc4cat:0000186	Attributes	A characteristic of an entity that is intrinsic to and cannot exist without the entity. (Properties, characteristics, qualities of things, states).

These top concepts are similar to those in top-level ontologies (BFO, SIO, DOLCE), to what is proposed in ANSI/NISO Z39.19-2005 (R2010), or what was proposed as minimal set of terms for a re-organization of AgroVoc.

Summary: Relation Types and Their Modeling

Relation Type	Modeling Approach	Example
IS-A	skos:broader/skos:narrower	Width IS-A dimensional property
HAS-A	skos:Collection membership	Substrate HAS-A {width, thickness}
PART-OF	External to attribute hierarchy	Window PART-OF reactor

Key principle: Keep the attribute vocabulary hierarchy pure by using skos:broader/skos:narrower exclusively for IS-A (taxonomic) relationships.