The criteria field varies according to whether it is the first field in the shipping file exactly matching the mode identifier. In that case, it is called the main criterion. If it is in subsidiary shipping lines matching the mode (with optional appended digits) then it is called a qualifying criterion. The difference is that the main criterion returns the basis for the calculation (i.e. weight or quantity) while the qualifying criterion determines whether the individual line may match the conditions.
The return must be one of:
table::field
notation.
[calc][item-list] ... [/item-list][/calc]
to create custom shipping routines.
IMPORTANT NOTE: The above only applies to the first field that matches the shipping mode exactly. Following criteria fields contain qualifier matching strings.