Knowledge in BEL is expressed as BEL Statements that are stored in BEL Documents. BEL Documents are structured text documents that contain BEL Statements, along with sufficient additional information to fully describe and process the document.
BEL Statements are generally defined as a semantic triple composed of a subject - predicate - object where the subject is always a BEL Term and the object can be either another BEL Term or a BEL Statement. The predicate is one of the BEL Relationship types describing the relationship between the subject BEL Term and the object.
BEL Terms are composed of BEL Functions and entity definitions referenced using BEL Namespace identifiers. Each BEL Term represents either an abundance of a biological entity, the abundance of human AKT1 for example, or a biological process such as cardiomyopathy.
Each BEL Statement can optionally be annotated with one or more references which express additional information about the statement itself such as the tissue, species, and cell line for example. Within a BEL Document statements can be associated with a set of Annotation Types that tag each statement and that can be used to describe the context in which the statement was observed and increases the richness of the knowledge being captured.