I'm not really sure what's definitive about it, but I think I read that it has more quests not found in the regular version, which I assume includes the 3ds.
The definitive version has a huge number of changes can't remember them all but here is what I do remember.
There is a wealth of small changes to improve quality-of-life, and streamline the overall experience. Of course, the biggest change to speak of is the upgraded graphics, which fall more in line with the look and style of Xenoblade Chronicles 2. There is also a wide array of other changes, which can be seen below.
Newly arranged soundtrack. Not every song has been re-recorded but most have. There's an option to switch between remastered and original soundtrack.
Combat arts now have markers that show when the player is in the right position for them, like Back Slash.
Auto-run has been added
The UI, in general, has been updated to make navigating menus easier.
Sidequests now have markers that lead players to required areas or items.
Colony 6 Reconstruction has been made easier, and money and materials for each category can be checked at any time on the menu.
Fashion equipment lets players change a character's clothing without sacrificing stats.
Casual Mode lowers difficulty even more.
Expert Mode allows players to stockpile XP and freely choose what level their characters are at, helping to make the game even more difficult.
An Event Theater allows players to rewatch cutscenes.
Time Attack Mode lets players take on new challenge battles with a time limit.
Anyone with save data for Xenoblade Chronicles 2 will get 100,000 Gold upon starting the game.
And then the big one the brand new epilogue, Future Connected. The epilogue takes place one year after the events of the main game, and it follows Melia and Shulk as they track down the imperial city of Alcamoth. The story takes roughly 15-20 hours to complete and takes place on the Bionis' Shoulder, an unused map from the original game that was cut content. Gameplay in Future Connected remains mostly the same, but Shulk and Melia are accompanied by Nopon party members and instead of Chain Attacks there's a new special attack system. Future Connected isn't an entirely new experience, but basically a continuation of the main game.
