Merchants have a set of Trades available to trade with a player. It picks a certain amount of trades from a trade tag to fill its trading inventory with. If less than the specified number of trades are present in the tag, it will use less trades instead. Note that modifying a trade after a Merchant has already received their trades won't change that Merchant's trade.

For Villagers, each profession has a tag dedicated for each level. It combines the id of the profession with its level name to determine the tag id. For example, a minecraft:butcher of level 2 (apprentice) uses the minecraft:butcher_apprentice trade tag to get its trades from. The only exception to these professions is the minecraft:nitwit, which has no trades and therefore doesn't use any tags either. Every time a Villager levels up, it picks two random trades from that level's tag and adds them to the Villager's trading inventory.

For a Wandering Trader, two trade tags are used to get trades from: minecraft:wandering_trader_regular and minecraft:wandering_trader_special. Every time a Wandering Trader spawns, it picks five entries from the minecraft:wandering_trader_regular tag and one from the minecraft:wandering_trader_special tag and adds them in that order to the Wandering Trader's trading inventory.