Game Case Study: Ico
In Chapter 2.11, "Player Toward NPC Chemistry Techniques," we looked in depth at the game Ico. (Please note: If you don't want to know the ending, please skip the next few paragraphs.)
Near the very end of the game, the boy you play obtains a magical sword that crackles with a kind of spiritual electricity. This is a Symbol of the Boy's Condition or Change in Condition. It symbolizes that he's attained a level of power: The demonic creatures that used to attack him now flee him and the sword. And it also symbolizes that he now belongs with the girl, for the spiritual electricity the sword exudes looks exactly like the mystical energy that the girl can wield when she needs to, and that has the same magical abilities. So the sword symbolizes two conditions: the boy's attainment of power and his attunement to the girl's soul.
Because the boy uses the sword to accomplish his final tasks, this is a Usable Symbol, serving double duty: working to deepen the emotional experience, but also playing a role in gameplay.