A fetcher finds Pimcore Objects and returns it. Per Default, Data Definitions only comes with one Fetcher: ObjectsFetcher, which returns all Objects of a Type.
To create your own Loader you need to implement Wvision\Bundle\DataDefinitionsBundle\Fetcher\FetcherInterface and add a new service
acme_bundle.data_definitions.my_fetcher:
class: AcmeBundle\DataDefinitions\MyFetcher
tags:
- { name: data_definitions.fetcher, type: my-fetcher }