The plugin ID.
There are no strict requirements as to the string to be used to identify the plugin, since discovery of the appropriate operation plugin to be used to apply an operation is based on the values of the 'toolkit' and the 'operation' annotation values.
However, it is recommended that the following patterns be used:
Type: string
public $id;
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https://api.drupal.org/api/drupal/core!lib!Drupal!Core!ImageToolkit!Annotation!ImageToolkitOperation.php/property/ImageToolkitOperation::id/8.1.x