Minecraft wiki entry for Loot_table, associated text: score—To query and use a scoreboard value.
 target: To choose which player name or entity UUID to query.
 type: Set to fixed to manually specify a player name or UUID. Set to context to use an entity from loot context.
 name: Included only if  type is set to fixed. Specifies the name of the player, or the entity's UUID (in hypenated hexadecimal format) whose score to query.
 target: Included only if  type is set to context. Specifies an entity from loot context to query the score of. Use this for the invoking entity, killer for the entity that killed the invoking entity, direct_killer for the entity that directy killed the invoking entity, or player_killer to select the killer only if they are a player.
 score: The scoreboard objective to query on the selected player name or UUID.
 scale: Optional. Scale to multiply the score before returning it.