This depends on the web method you're using:

• Using one of the "GetElementsForXXXCached" methods returns only the elements which have changed since the specified unix timestamp. This is interesting when using polling to keep an updated list of elements. LastChangeUTC then contains the timestamp of the last polling time when the last change (different name, severity, timeout) was detected.
• For all other methods, LastChangeUTC contains the timestamp of when this information was received from the DMA. So this property is only interesting when using it in combination with one of the "cached" web methods.