Mark,

FetchSize controls HOW result set is fetched (at once or one batch of records at a time).
FetchAsNeeded controls WHEN additional batches are fetched, this property is taken into consideration only if FetchSize>0.
FetchAsNeeded=.F. - fetch is done during idle time and when it is needed
FetchAsNeeded=.T. - fetch is done only when it is needed

Thanks,
Aleksey.