USM-eM is a competitive-oriented mod for Smash 3DS.

It stands for Unnamed Smash Mod (USM) / Unnamed Smash Melee-esque Mod (USM-eM).

Changes include character and faster-paced gameplay changes, as well as a handful of extras like stages, sound effects, an OST, skins, UI, and more.

Every single character will have balance changes eventually to increase their viability, and currently every character is affected by global changes such as increased gravity, altered ledge mechanics, dash dancing, and more. As of yet, only a few characters have been overhauled.

If you stumbled upon this page in search of the Nintendo 3DS userland exploit, this is an unrelated game specific mod.
You can (and should!) use USM to play USM, of course. It’s free and of no risk to modify your console.