14 files declare their use of FieldConfigInterface in Zircon Profile 8

comment.module in core/modules/comment/comment.module

Enables users to comment on published content.

config_translation.module in core/modules/config_translation/config_translation.module

Configuration Translation module.

field.module in core/modules/field/field.module

Attach custom data fields to Drupal entities.

field.purge.inc in core/modules/field/field.purge.inc

Provides support for field data purge after mass deletion.

FieldConfig.php in core/modules/field/src/Entity/FieldConfig.php

Contains \Drupal\field\Entity\FieldConfig.

FieldConfigEditForm.php in core/modules/field_ui/src/Form/FieldConfigEditForm.php

Contains \Drupal\field_ui\Form\FieldConfigEditForm.

FieldConfigListBuilder.php in core/modules/field_ui/src/FieldConfigListBuilder.php

Contains \Drupal\field_ui\FieldConfigListBuilder.

image.module in core/modules/image/image.module

Exposes global functionality for creating image styles.

ImageFormatterBase.php in core/modules/image/src/Plugin/Field/FieldFormatter/ImageFormatterBase.php

Contains \Drupal\image\Plugin\Field\FieldFormatter\ImageFormatterBase.

MigrateBlockContentBodyFieldTest.php in core/modules/block_content/src/Tests/Migrate/MigrateBlockContentBodyFieldTest.php

Contains \Drupal\block_content\Tests\Migrate\MigrateBlockContentBodyFieldTest.

MigrateFieldInstanceTest.php in core/modules/field/src/Tests/Migrate/d7/MigrateFieldInstanceTest.php

Contains \Drupal\field\Tests\Migrate\d7\MigrateFieldInstanceTest.

MigrateNodeTypeTest.php in core/modules/node/src/Tests/Migrate/d7/MigrateNodeTypeTest.php

Contains \Drupal\node\Tests\Migrate\d7\MigrateNodeTypeTest.

views.module in core/modules/views/views.module

Primarily Drupal hooks and global API functions to manipulate views.

views.views.inc in core/modules/views/views.views.inc

Provide views data that isn't tied to any other module.