From the Super Mario Wiki
Experience Points, usually just referred to as Experience, Exp., EXP, or XP, are points used by characters in RPGs such as Super Mario RPG: Legend of the Seven Stars, the Mario and Luigi series, and the Paper Mario series. When characters defeat enemies in battles, they earn Experience Points. When a character earns enough Experience Points past a certain point, they Level Up, earning stat increases such as HP, attack power, and so on. As characters gain levels, the amount of Experience Points needed to reach levels increase, making it more difficult to level up. In general, enemies encountered at the beginning of a game drop less Experience Points than those found later on. Bosses and rarely-encountered enemies can drop a significant amount of Experience Points when defeated. When a character reaches their maximum level, Experience Points do nothing for them, and whatever Experience Points collected after that are not distributed to other characters.
Super Mario RPG: Legend of the Seven Stars
In Super Mario RPG: Legend of the Seven Stars, Experience Points earned in battles are divided up between the party members (halved if there are two, in thirds for three), and are displayed as such on the victory results screen: if the player's party earns 30 Experience Points from a battle, "10 Exp." will be displayed. Characters can also receive Experience Points from battles, even if they did not participate in them; they would earn the same amount as those in the player's current party. The player can purchase the Exp. Booster Accessory to double the amount of Experience Points only for the party member wearing it. If the player gets a Game Over in battle, the player is sent back to their last Save Point while still retaining any new levels, Experience Points, Coins, and items gained. The player also has a chance to double their Exp. earned at the end of a battle from the Wheeere's Yoshi?! minigame if they successfully get the Baby Yoshi from one of the three eggs shuffled around.
A party member can collect a maximum of 9,999 Experience Points, bringing them to the maximum level of 30. Experience Points are still divided up regardless of how many party members have already reached this level.
For a listing of how many Experience Points are needed to advance levels for each character, see: Super Mario RPG: Legend of the Seven Stars level up progressions
Paper Mario series
Mario & Luigi series
In the Mario & Luigi series, the total Experience Points earned in battle is applied to all characters, effectively doubling, tripling or quadrupling the amount actually earned. If a character is KO'd when a battle is over, they do not earn any Experience Points at all.
Puzzle & Dragons: Super Mario Bros. Edition
In Puzzle & Dragons: Super Mario Bros. Edition, XP earned in battles are divided up between the teammates on the player's active team: the less teammates the player has on their team, the more XP each teammate earns. If the XP divided between the teammates ends at a decimal point, the value is rounded up. As with Super Mario RPG: Legend of the Seven Stars, XP is divided up between all members of the player's team regardless of how many have already reached their maximum levels.
The player can also provide XP for their teammates by depowering other teammates in the player's Ally Box for their XP in the Power Up spot at Toad Houses. Depowering teammates removes them from the player's Ally Box and up to five allies can be depowered at a time. A teammate can provide more XP for another the higher their level is, with their base XP value multiplied by their current level. Teammates getting an ally's XP receive 50% more if their main attributes match. Coin Coffers and King Coin Coffers provide a significant power up for teammates using this method. Occasionally, a teammate may earn twice as much XP from another teammate than normal.
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