Items sets allow you to link multiple items together to make them stronger when worn at the same time. Item sets give extra stats to the player if he wears enough items from the same item set. The config file is pretty self explanatory; here is a config sample.
ARCANE: name: '&2Arcane Set' bonuses: '3': magic-damage: 20 '4': max-mana: 30 potion-speed: 1 lore-tag: - '&7Arcane Set Bonus:' - '&8 +20% Magic Damage' - '&8 30 Max Mana' - '&8 Permanent Speed I'
Set bonuses can be displayed in the item lore using the lore-tag option. You can really put anything in that list, but I like to precisely give the amount of additional stats the item gives. The bonuses section defines what bonuses the item set gives. The subsections with numbers correspond to the stats the item set grants for each amount of items the player wears.
Special Full Set Bonuses
Set bonuses can also grant permanent potion effects to the player. You need to specify the potion effect name (names can be found here) and the potion level which needs to be an integer.
SPEED: name: '&2Speed Set' bonuses: '4': speed: 1
Last but not least, you may also setup abilities as full set bonuses. Pretty easy to setup since it's the same format as with item abilities. In the following example, any player who holds at least 2 items from the Hatred Set will be temporarily granted a Life Ender ability which will trigger when hitting any entity. The ability has a 30sec cooldown.
HATRED: name: '&cHatred Set' bonuses: '2': ability-1: type: LIFE_ENDER cooldown: 30 mode: ON_HIT
Potion effects as full set bonuses: in this example, wearing 4 armor pieces from the Arcane Set applies permanent Speed I to the player.
ARCANE: name: '&2Arcane Set' bonuses: '4': potion-speed: 1