Ideally the last-modified date of a document would always be used to identify if it has changed (see IgnoreLastModifiedDate property), however web-servers do not always return this information. Further, dynamic pages do not have 'last-modified' dates, since they are dynamic.

The byte size of a document can however be used to identify change with reasonable accuracy. If a document has a different file size, then it has definitely changed. Of course, it is also possible for a document to change, but have the same size (exactly the same number of letters are deleted as added, for example), so this is not 100% accurate.

Despite not having 100% accuracy, it may be desirable to forgo accuracy for performance, by setting this to true.