User blog comment:Cloudthehedgehog12/Super Smash Bros. 4 Ideas/@comment-2135507-20110601012551/@comment-11644326-20110601013432

Unlockable generally means you unlock them as you move through the story- Ex. how you unlocked various characters in Sonic Advance 2, secret generally means you have to complete a specific task or get certain unlockables to get them, ex. how you need to collect gold emblems in Sonic Adventure to get Metal Sonic as a skin.