Quest set mode allows you and 1-3 friends to play though all the quests in one of the sets of your choice. The differences between normal Co-op and Quest Set Co-op are thankfully pretty simple:

• You don't have to remake the room every time you want to change quests, it automatically goes to the next quest in the set.
  
• Depending on the quest set you select, you get credit for clearing it. Example: Clearing the first bonus quest in Quest Set Co-op counts as if you cleared it by yourself in single player mode.
  
• This one is the most important for people who want to play Co-op: You can play every single Free Play quest in 4 player mode!

There are both good things and bad things about Quest Sets. I won't be mentioning anything about Internet problems or the like for this section, so let's start the naming!

Advantages:

• Anything cleared in Quest Set counts as clearing in the mode it came from.
  
• You can access all the quests from the word go no matter how early or late in the plot you are.
  
• EXP, G, and items are still earned.
  
• Playing the 1/2 players only quests with 4 players.
  
• Even if you can't get friends or random players, you can play with CPUs as well although you need to set number of maximum players to 3 or 4. (And unlike the 3DS version, you don't need 2 Human players!)

Disadvantages:

• 2P Freeplay quests are the only multiplayer quests you can't get credit for using Quest Set mode. You have to use normal Co-op for that.
  
• Story quests are NOT included, so you still have to complete those quests if you want to unlock the characters earned from those quests.
  
• You can't start from a quest in the middle of a set. Example: If you disband the room at Quest 3/6, you have to start from Quest 1/6 the next time you challenge that Quest Set, so prepare some food and drink just in case.

Once you select a character after selecting Co-op Quest Set you will be asked to choose how many player slots you want (I know it's asking for which quest set, but it's really asking how many players you want) then, select which Quest set you want to do.

Long Quest Sets: Freeplay All, 1P Extra Quest All, 2P Extra Quest All

These 3 sets are every single quest in that catagory. Make sure you have some food and drinks...and time, ready for these if you play to do them. Freeplay are all the Freeplay quests in a row, 1P Extra Quests are the same deal, while the 2P Extra Quests are the normally 2P only Extra Quests.

Middle Quest Sets: 1-4 Extra Quest *1-5

These quest sets are, for the 1P and 2P quest sets, taken from the them, but put into 6-7 quest chunks, making them way more easier to playthough all in one sitting. The 3P and 4P Extra quests are thankfully pretty simple, You play all of them in a row (Since there are only a few of them) at that difficulty.

Short Quest Sets: Corpse Party, Love Villagers Rescue Plan

As you might have guess by the word short, these are really quick, being only 3 stages long. I'll admit, I don't know much about the quests, since I don't think they were in the 3DS version? I never did play Multiplayer when playing it...

Hopefully this guide helps explain how Quest sets work, and you understand how to use them. Feel free to make any comments to either help explain stuff that I missed or if you have any questions!