A combination of not shown in the hierarchy, not saved to to scenes and not unloaded by The object will not be unloaded by Resources.UnloadUnusedAssets.
This is most commonly used for objects which are created by scripts and are purely under their control.
using UnityEngine;
using System.Collections;
public class ExampleClass : MonoBehaviour
{
// Creates a material that is explicitly created & destroyed by the component.
// Resources.UnloadUnusedAssets will not unload it, and it will not be editable by the inspector.
private Material ownedMaterial;
void OnEnable()
{
ownedMaterial = new Material(Shader.Find("Diffuse"));
ownedMaterial.hideFlags = HideFlags.HideAndDontSave;
GetComponent<Renderer>().sharedMaterial = ownedMaterial;
}
// Objects created as hide and don't save must be explicitly destroyed by the owner of the object.
void OnDisable()
{
DestroyImmediate(ownedMaterial);
}
}