## FAQ

### How do I type nodes in a graph with concepts that are not in the Biolink Model?

Each node in a knowledge graph can be typed using the slot `category`

.

In addition to `category`

, one can type a node using the `rdf:type`

and `rdfs:subClassOf`

predicates.

### How do I type edges in a graph with concepts that are not in the Biolink Model?

Each edge in a knowledge graph can be typed using the slot `category`

.

In addition to `category`

, one can type a node using the `rdf:type`

.

### How do I add properties that are not in the Biolink Model

Each node and/or edge can have properties that are outside of Biolink Model.

Alternatively, for a more structured representation it is recommended to use the class `Attribute`

to represent the property and link a node/edge using the `has attribute`

slot.

### What is the serialized form of Biolink Model?

Refer to [Working with the Model[working-with-the-model.md] for an example on how a Biolink Model graph can be represented as labelled property graphs and RDF graphs.

### What is the difference between `predicate`

, `relation`

, `category`

?

`predicate`

is an association slot and must have a value from the`related to`

hierarchy`relation`

is an association slot and can have a value from any external ontology, controlled vocabulary, thesauri, or taxonomy`category`

(or`rdf:type`

) is a slot and must have a value from the`named thing`

or the`association`

hierarchy.