References include information about external sources - links, citations, mentions, names, and titles - and point out relations between informational objects:
- notes recognizing a source of information;
- names of the authors;
- titles of specific publications containing authoritative facts;
- definitions, standards, and their origination;
- passages from publications,
- mentioning, embedding, etc.
Definitions are statements of the meaning of a term:
- Intensional - give the sense of a term by naming properties that a thing must have in order to be part of the set.
In logic and mathematics, an intensional definition gives the meaning of a term by specifying necessary and sufficient conditions for when the term should be used.
- Extensional - list the objects that a term describes).
Extensional definitions are used when listing the members of a set tells the questioner enough about the nature of that set, or gives more applicable information than other types of definition.
- Ostensive - convey the meaning by pointing out examples of one or more members of a set, but not necessarily all.
A term may have many different senses and multiple meanings, and thus require multiple definitions. Definitions and axioms are the basis on which all of modern mathematics is constructed.
Standard is a criterion, an acknowledged measure of comparison for quantitative or qualitative value. Standards can be:
- Objects that under specified conditions represents the unit.
- Degrees or levels of requirement or excellence.
- Sets of specifications that are adopted to allow compatibility.
Knowledge - useful information, facts, methods, tips, tutorials - presentations of concepts, abstract ideas, general notions, conceptions, abstractions, theories, hypotheses, and beliefs and convictions based on factual information and experience.