Minecraft wiki entry for Development_versions, associated text: General syntax: /loot <target> <source>.
Sources:
fish <loot table id> <fishing location> [tool <item>|mainhand|offhand]
Uses fishing context.
loot <loot table id>
Uses loot chest context (can be also used for advancement awards and cat gifts).
kill <entity selector>
Simulates entity drops.
mine <mining location> [tool <item>|mainhand|offhand]
Simulates block drops.
Targets:
spawn <position>
Drops in world.
replace
Works similar to /replaceitem. If count is missing, command will try to place all returned items. If count is higher than number of items, remaining slots will be cleared.
entity <entity selector> <start slot> [<count>]
Replaces a range of slots.
block <position> <start slot> [<count>]
Replaces range of slots.
give <player selector>
Inserts items into player's inventory (similar to /give).
insert <position>
Inserts items into container (similar to ⇧ Shift+left-click).