When an operational report is executed an entry will be created in the report archive. The execution of an operational report in IFS Cloud is equal to assembly of data for a specific report given some report parameters. The entry created in the report archive is a record for this report execution and holds both the data assembled as well as information about who triggered the execution, when, with what parameters and so on. The report archive serves as a history/record of previously executed reports (i.e. previously assembled data). This means that you can refer to the report archive if you want to format the same data again using a different layout or some different parameters, opposed to running the report again (triggering a new collection/assembly of data - data that might have changed since the last time the report was executed).
In order to prevent report archive to just grow and grow there are scheduled cleanup jobs that removes expired items from the archive. When an item is created in the archive it get's a expiration date (the default life span is configurable). The expiration date can also be changed on individual items both manually and programmatically.
In addition to the Report Archive (which holds the assembled data of executed reports), there's a PDF Archive that keeps a historic record of reports (i.e. records in the Report Archive) that have been formatted (i.e. assembled report data for which a layout and some formatting parameters have been applied in order to produce a formatted PDF document. See PDF Archive for further details.