That is ratio between number of rows in cache located below or above visible rows in grid and number of visible rows. When the threshold is reached either on the top or on the bottom edge of the visible rows, then an asynchronous request for more rows from server is triggered.
Target Platforms: Windows 10, Windows 8.1, Windows 8, Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2