The callout element signifies the invocation of code that is not part of IEG to perform some validation or other operation for the script execution, with access to the Datastore provided to the function through the appropriate identifiers.
Callouts can be placed anywhere question pages can exist in the script, except before the first question page. They will invoke a custom function specified in the expression before moving to the next page. Note that if a page is accessed from a summary page link and is followed by callouts, the callouts will get invoked unless the 'skip-to-summary' attribute has been set to true on the summary link.