Fill height and Fill flow
Fill height is a measure of the vertical depth of the sink. Sinks that are deeper than the Fill height specified will not be filled and will be left unchanged. Sinks that can be filled with the Fill height specified will be filled.
Consider a lake formed by a dam where the lip of the dam is 50 meters above the height of the lake. The lake is a sink until the level of the lake rises up above the lip of the dam and can spill over past the dam downstream. The lake is a sink with a vertical depth of 50. The Fill height required to fill it is 50. At any Fill height values of 50 or greater, the lake as a sink is filled. At any Fill height less than 50 the lake is unfilled and remains as a sink.
If we use a very large value for Fill height, such as 20000 (a very big height difference whether we are measuring in feet or meters) that will be enough to fill in any sink.
Fill flow is a measure of the areal size of the sink. A sink is a closed drainage basin, the total flow of which is found by assuming one unit of water falls on every pixel within the basin. To fill the entire basin we must have at least that much flow available to fill the basin. If a sink is 1000 pixels in areal size we must have at least 1000 in Fill flow available to fill it.
If we choose 1000 as the value for Fill flow that will be enough to fill all sinks that are 1000 pixels in size or less. Sinks that are larger than 1000 pixels in size will be too big to fill with the Fill flow we have specified. A Fill flow specification of 500 would be enough to fill a sink that is 400 pixels in size, but it would not be enough to fill a sink that is 1000 pixels in size and thus requires a minimum of 1000 in flow to be filled.
Since it usually is easier to note the height of a dam or other obstruction that it is to compute the areal surface of a drainage basin, in many cases it will be easier to simply use a Fill height value when filling in sinks created by dams. Fill flow may be more useful when filling in small sinks in undulating terrain where we are interested in larger effects and do not care about highly local sinks that are, in comparison with larger watershed areas, mere puddles.