The list of suppliers that appear for your good or services, is directly related to your good and services catalogs. The suppliers assigned to each of your list of services will be displayed here.
Example: You offer service X, and on your catalog you have indicated that this service can be provided by your supplier A or E, then you will have the choice to select between A and E only. To learn how to assign suppliers to your goods or services look for How to Add products or services? (3 methods)