Coordinate differentials are assumed to be anything that is a coordinate name with a d in front of it, such as dt or dx if t and x are coordinates. A coordinate differential is defined by the index region and coordinate region of the first grid which uses its coordinate. For instance, if the following coordinate system and grids are defined:
rect coordinates t,x,y
uniform rect grid g1 [1:Nx][1:Ny] {xmin:xmax} {ymin:ymax}
uniform rect[x] grid g2 [1:N] {min:max}
then dx will be defined from g1 by 
even
though g2 may have a different coordinate spacing.