The mesh order is used by the simulation engine to select which material to use when two materials
overlap in the simulation region.
For example, here I have a sphere made of material 1 and a rectangle which is made of
material 2 and overlaps with the sphere in some areas in a 2D simulation.
Also, from the geometry settings, I have given the sphere material a mesh order of 2 and
the rectangle a mesh order of 1.
Always remember that in the case of overlap between two materials, the material with lower
mesh order will get a higher priority.
In this example, the rectangle with a mesh order of 1 will take precedence over the sphere
which has a mesh order of 2.
The Areas that overlap (in this example domain 1) are assigned the material properties of
the higher priority object (in this case material 2).
In the event that both overlapping objects have the same order, the mesh order will be
inferred from the Objects Tree.
Objects at the bottom of the tree will take priority over objects at the top of the tree.