The QueryItems event, which is raised whenever new records are required, allows the next batch of records to befetched from the data source and provided to the grid. New items are provided to the grid by calling the EndQuery method, which is exposed by the AsyncQueryInfo instance received in the event arguments of the QueryItems event, with an array of items. The StartIndex property of the AsyncQueryInfo indicates the index in the data source at which to start retrieving the new items while its RequestedItemCount property represents the number of new items that should be returned. The current state of the collection view (e.g., sorting and filtering) must always be taken into consideration when returning new items. If the EndQuery method is called with an array that does not match the requested item count, the collection view will automatically refresh. If the array contains a a null reference (Nothing in Visual Basic), an exception will be thrown.

Reference

AsyncQueryInfo Members
Xceed.Wpf.DataGrid Namespace