- *
- * If parent is empty, any further processing of the property is skipped - thus,
- * the next plugin (migration) will not be run.
+ * If 'parent' is empty, any further processing of the property is skipped and
+ * the next process plugin (migration_lookup) will not be run. Combining
+ * skip_on_empty and migration_lookup is a typical process pipeline combination
+ * for hierarchical entities where the root entity does not have a parent.