It is possible that at some point historical data might be modified on the server, and you are interested in these changes. In this case you would use readModified function. This function returns the timestamps at which the data was modified and the value before that modification. If readRaw, readAtTime, or readProcessed returns a quality value of OPCHDA_EXTRADATA, it indicates that the item in question has been modified and more information can be retrieved using readModified. By providing the function with a list of items that you are interested in and the time range over which you would like to query for changes, you can retrieve any changed data items. This function operates similarly to readRaw, but only modified data is returned.