Expert Mode

Expert Mode is a gameplay option in Xenoblade Chronicles: Definitive Edition. Contrary to what the name might imply, it is not a "hard mode", and has no effects in battle at all. Instead, it provides a means to avoid over-levelling, thus allowing the player to do as many quests and other side content as desired without having to worry about combat becoming trivially easy. It can also be used to intentionally under-level for challenge runs or similar.

When enabled, Expert Mode has two main effects.
 * EXP gained from completing s, discovering s and s, and earning s is not applied to characters immediately. Instead, it is stored aside for use later. EXP gained from enemies is applied as usual.
 * The player can directly set the of each party member individually. Levelling up requires and spends stored EXP, while levelling down does the opposite. The conversion between levels and EXP is lossless until level 99, at which point excess EXP does not matter anyway. Disabling Expert Mode does not automatically convert unused EXP to levels; the EXP remains unused and inaccessible until Expert Mode is re-enabled.

Levelling down a character does not cause them to lose arts or affinity coins gained via levelling up, and such things are not double-dipped by regaining previously-achieved levels. New party members are set to the current average level of the rest of the party and do not receive any stored EXP, so there is a risk that everyone else is capable of going back up to 20 but the newcomer is stuck at 10.

There is nothing preventing the player from enabling both Expert Mode and Casual Mode at once, allowing for decidedly wacky situations such as being able to defeat es at very low levels.