Minecraft wiki entry for Development_versions, associated text: Drops items in various contexts.
Syntax: /drop <target> <source>.
Sources:
award: advancement award context.
fish: fishing context.
loot: loot chest context.
kill: entity drops.
mine: block drops.
Targets:
world: drops in world.
player: puts in player inventory.
entity: sets entity slots like replaceitem.
block: puts (distribute, as if shift-clicked) or replaces/merges (insert) items in container.