It should be specified as a fraction of the cells not an absolute number, because the user may have many matrices of different sizes they wish to compare and of course the source and target may be different sizes (see below). I.e. I would rather specify: '0.8' meaning e.g. 800 genes of the 1000 in the matrix must be expressed for a cell to be valid, than '800' directly. Then I can leave that parameter alone even if I look at a matrix with 10000 cells or a hundred cells and get an equivalent filtering.