Items

For menus, the Standard item format is respected, to which a few extra settings are added.

# item config example
item:
# standard settings
  material: GOLD_INGOT
  custom-model-data: 12
  amount: 1
  name: '&aExample'
  lore:
    - '&fLine1'
    - '&fLine2'
# menu settings
  slot: 1
  slots: [1,2,3,4]
  click-actions: ...

Slot or Slots

If you want to have the item in a specific slot you can either use the slot setting directly or the slots with a single element inside the list.

Due to the complex nature of menus these days, caused by the usage of texture packs that have buttons spanning on multiple slots, you can use the slots setting to set the same item in multiple places.

You technically can use both settings at the same time, but it just looks silly.