System entities are pre-built entities provided by Dialogflow that facilitate handling the most popular common concepts.
You can see a full list of Dialogflow's system entities in System Entities. Below are some examples of the different types of system entities.
These system entities have reference values. For example,
@sys.date matches common date references such as "January 1, 2015" or "The first of January of 2015" and returns a reference value in ISO-8601 format:
These entities have no reference value. For example,
@sys.color matches most popular colors and returns the matched color as it is without mapping it to any reference value. For example, shades of red, such as "scarlet" or "crimson", won't be mapped to "red" and will return their original values "scarlet" and "crimson".
These entities can contain other entities with aliases and return object type values. For example,
@sys.unit-currency is meant for matching amounts of money with an indication of a currency name like, "50 euros" or "twenty dollars and five cents". It returns an object type value consisting of two attribute-value pairs: