Carriers can be attached to products, if a particular carrier is only available for a particular product.
For example, if you sell musical instruments like flutes, tambourines and so on, and you have a piano in stock, you obviously can't use normal post or your carrier's standard services to deliver this product. It is likely that you will have to use a dedicated hauler for this type of product.
To do this, just go to Catalog then Products and edit the product to which you want to attach your carrier.
You can choose to have the current product only be shipped by a selection of carriers. If no carrier is selected then all the carriers will be available for customer orders.
From the Product page, click on the Shipping tab and at the bottom of the form are all of the carriers that have been created. Just select the one that will be able to deliver the product. Then save the changes to the product.