A value in the range (0.0, 1.0) that specifies the fractional
point at which the original stream should be split. The actual
split point is evaluated on pre-filtered rows, so if a filter
is provided, then there is no guarantee that the division of
the rows between the new child streams will be proportional to
this fractional value. Additionally, because the server-side
unit for assigning data is collections of rows, this fraction
will always map to a data storage boundary on the server side.