fmAll

All result set records are fetched automatically by ADO.Net provider at the dataset Open call. On a big resultset it may take a time, but then navigation through a dataset will be fast, because all records will be stored in an ADO.Net internal data storage.

fmOnDemand

The records are fetched automatically by ADO.Net provider, when they are demanded by the navigation or other dataset methods. At each demand ADO.Net provider will fetch one or more record sets, containing RowsetSize records. This mode allows to open big result set without a significant delay, like a fmAll mode. But total time of fetching of the all records will be more than in fmAll mode.