The default value(s) allows you to apply the same equation or value for all indices of a non-scalar block.
For example, an expression Concentration is material dependent; it’s equation is “Mass/Volume” and Mass is a material dependent parameter. Thus, you must specify an equation for each material. Instead of having to enter the same equation for each material, you can instead enter the equation in the default field. The equation will then be used for all materials.
The default value only applies where no other value/equation has been written, allowing you to override the value for specific indices.
For two or three dimensional blocks, you can also assign default values for each list index list. For more information see editing values.