16 files declare their use of SelectInterface

CommentSelection.php in drupal/core/modules/comment/lib/Drupal/comment/Plugin/entity_reference/selection/CommentSelection.php
Contains \Drupal\comment\Plugin\entity_reference\selection\CommentSelection.
Condition.php in drupal/core/modules/field_sql_storage/lib/Drupal/field_sql_storage/Entity/Condition.php
Definition of Drupal\field_sql_storage\Query\ConditionSql.
ConditionAggregate.php in drupal/core/modules/field_sql_storage/lib/Drupal/field_sql_storage/Entity/ConditionAggregate.php
Contains \Drupal\field_sql_storage\Query\ConditionAggregate.
FileSelection.php in drupal/core/modules/file/lib/Drupal/file/Plugin/entity_reference/selection/FileSelection.php
Contains \Drupal\file\Plugin\Type\selection\FileSelection.
node.module in drupal/core/modules/node/node.module
The core module that allows content to be submitted to the site.
NodeSelection.php in drupal/core/modules/node/lib/Drupal/node/Plugin/entity_reference/selection/NodeSelection.php
Contains \Drupal\node\Plugin\Type\selection\NodeSelection.
PagerSelectExtender.php in drupal/core/lib/Drupal/Core/Database/Query/PagerSelectExtender.php
Definition of Drupal\Core\Database\Query\PagerSelectExtender
SelectionBase.php in drupal/core/modules/entity_reference/lib/Drupal/entity_reference/Plugin/entity_reference/selection/SelectionBase.php
Contains \Drupal\entity_reference\Plugin\entity_reference\selection\SelectionBase.
SelectionBroken.php in drupal/core/modules/entity_reference/lib/Drupal/entity_reference/Plugin/Type/Selection/SelectionBroken.php
Contains \Drupal\entity_reference\Plugin\Type\Selection\SelectionBroken.
SelectionInterface.php in drupal/core/modules/entity_reference/lib/Drupal/entity_reference/Plugin/Type/Selection/SelectionInterface.php
Contains \Drupal\entity_reference\Plugin\Type\Selection\SelectionInterface.
Tables.php in drupal/core/modules/field_sql_storage/lib/Drupal/field_sql_storage/Entity/Tables.php
Definition of Drupal\field_sql_storage\Entity\Tables.
tablesort.inc in drupal/core/includes/tablesort.inc
Functions to aid in the creation of sortable tables.
TermSelection.php in drupal/core/modules/taxonomy/lib/Drupal/taxonomy/Plugin/entity_reference/selection/TermSelection.php
Contains \Drupal\taxonomy\Plugin\entity_reference\selection\TermSelection.
user.module in drupal/core/modules/user/user.module
Enables the user registration and login system.
UserSelection.php in drupal/core/modules/user/lib/Drupal/user/Plugin/entity_reference/selection/UserSelection.php
Contains \Drupal\user\Plugin\entity_reference\selection\UserSelection.
ViewsSelection.php in drupal/core/modules/views/lib/Drupal/views/Plugin/entity_reference/selection/ViewsSelection.php
Contains \Drupal\views\Plugin\entity_reference\selection\ViewsSelection.