The term "Dataset" in ECL explicitly means a "physical" data file in the supercomputer (on disk or in memory), while the term "Record Set" indicates any set of records derived from a Dataset (or another Record Set), usually based on some filter condition to limit the result set to a subset of records. Record sets are also created as the return result from one of the built-in functions that return result sets.

A Record Set Definition is defined as any Definition whose expression is a filtered dataset or record set, or any function that returns a record set. For example, the following are all Record Set Definitions:

FloridaPersons    := Person(Person.per_st = 'FL');
OldFloridaPersons := FloridaPersons(Person.Age >= 65);