What Are Field Sets?
The Default field set includes standard built-in generic and repository-specific CES fields (see Available System Fields).
More connectors progressively come with a field set import file defining fields for all default metadata available for a repository type. The field set import files are available from the [CES_Path]\Bin\ folder and the file name is in the following format: Coveo.CES.CustomCrawlers.[ConnectorName].FieldSet.xml. When a field set import file is available for a connector, it is a best practice to create a field set starting with importing the content of this file to easily get all default fields that can immediately be used to create facets, sort criteria, and display fields (see Exporting and Importing a Field Set).
You can create custom field sets that are copies of the Default field set (see Adding a Field Set) but in which you can modify the properties of built-in fields and to which you can add custom fields to map specific metadata values from one or more repositories (see Adding or Modifying Custom Fields). You then assign a custom field set to one or more sources for which it was created (see Modifying the Field Set Used by a Source).
Example: You index a Sitecore website and want to index several business logic metadata from the website. You create a custom field set named My Sitecore Fields to which you add a bunch of custom fields for each of the Sitecore metadata that you want to index. You associate the custom field set to your Sitecore source.