You can see only one character has been saved to instances. Vocah (my main) has done Gundrak and Drak'Tharon Keep on Heroic difficulty.
The Obsidian Sanctum, Naxxramas and The Nexus were done previously, but aren't currently saved. You can choose to hide all previous instances (that aren't also current) or hide them individually. It's completely up to you!
You can also re-order the instances.