|target||The object to mark as dirty.|
target object as dirty. (Only suitable for non-scene objects).
NOTE: ''Prior to Unity 5.3, this was the primary method of marking objects as dirty. From 5.3 onwards, with the introduction of Multi-Scene Editing, this function should no-longer be used for modifying objects in scenes. Instead, you should use Undo.RecordObject prior to making changes to the object. This will mark the object's scene as dirty and provide an undo entry in the editor.''
If you are modifying files within a scene and specifically do not want to add an undo entry for your modification, you should use EditorSceneManager.MarkSceneDirty
If you are using a custom editor to modify serialized properties on a component or an asset, you should be using [pSerializedObject.FindProperty]], SerializedObject.Update, EditorGUILayout.PropertyField, and SerializedObject.ApplyModifiedProperties. This will mark the modified object as 'dirty' and create Undo states for you.
Therefore the only remaining situation in which you should use this function is if you are modifying non-scene objects via some other means, and specifically do not want create an undo entry for your modification. This is rare, and unless you're very sure about requiring this, you probably shouldn't be using this command!
Unity internally uses the dirty flag to find out when assets have changed and need to be saved to disk.