Within the context of Cloud API, an apiconfig file is a glue file that maps both resources to Gosu files. The apiconfig file is also where default sort orders for collections are defined.
The only time you need to configure an apiconfig file to connect resources to Gosu files is when you are completing a configuration for CRUD endpoints created by the REST endpoint generator. For more information, see Configuring glue and impl classes for generated endpoints.
For more information on defining default sort orders, see Sorting a collection.