The profiles section of the system allows you to divide up your shipping into different groups in the case you need to have different shipping rules for some shipments (i.e. customer specific rules such as billing). By default, the system is set up with one profile set to the address on file from your contract. You can add/edit any profiles you want in the system as long as you are signed in with an Admin user.
Here's our documentation on the profiles: Profiles
Once you have the carriers set up in the system, you can set up the carriers and services you want allowed for the profile here.