External Sets are definitions of data structures that are not stored in the database but are only relevant while a job is being executed.
In particular, they are definitions of:
•The data structure of external files used during import and export operations.
•The data structure of parameters passed to, or returned by, executed jobs.
In the pre-XML technology of Import Tasks, they are also used to define temporary, table-like structures in which external data is collected and merged prior to being mapped to database tables. This usage is being replaced by SQL Tasks that handle XML and XSLT.
External sets are defined much in the same way as Tables or Logical Views. They have External Set Elements in the same way that Tables and Logical Views have Columns. One way of defining an External Set is by writing an SQL SELECT statement, in much the same way as a Logical View is defined by its SQL SELECT statement.