The slot data structure is how Minecraft represents an item and its associated data in the minecraft protocol
The structure consists of at least a short, which gives the item/block ID . A value of
-1 signifies that the slot is empty, and no further data follows.
If the block ID is not
-1, three more fields follow. These fields are a byte (item count), a short (item damage) and another short (nbt data length).
If the final short is
0xffff, there is no NBT data, and no further data follows. Otherwise that short is the start of the nbt blob.
The format of this data is as follows:
COMPOUND: '' LIST: 'ench' COMPOUND SHORT: 'id' SHORT: 'lvl' END COMPOUND ...etc END END
Each of the inner, untagged COMPOUNDs represents an enchantment, with its ID and level given as child SHORT elements.
ff ff | empty slot 01 16 01 00 00 ff ff | a diamond pickaxe 01 16 01 00 00 00 04 CA FE BA BE | a diamond pickaxe with (made-up) NBT data