User defined shipping methods allow to set simple rules to calculate the order shipping cost based on the customer's location, order weight and number of items. Each user defined method can be made available to customers when their order total weight fits some particular limits; for each

destination zone in your store you can define a cost calculation rule that consists of a per order, per item and per weight unit rates. So, the final cost is calculated by the formula:

Code:

Shipping cost = per order rate + (number of items in order × per item rate) + (total weight × per weight unit rate)

- all 3 rates are always used.

Such system allows for simple table rate shipping cost calculations or some very interesting shipping cost schemes:

http://kb.ecwid.com/w/page/15853310/Shipping#Examples
What exactly do you want to do with the help of the user defined shipping methods?