Exploration

There are many secrets to be found around the Retroverse, secrets which will only be revealed to the diligent. To aid in giving an explorer a sense of how far he has progressed, he can see the percentage of the Retroverse he has explored in his 'score'. It is also rumoured that if the explorer takes the time explore with open eyes, he will earn some experience when visiting new locations other than the outerworlds for each of the six worlds.

Exploration gives experience. The greater your exploration percentage, the greater the experience you receive.
Note: this means you cannot be in brief mode, nor during a running battle.