Fluid ingredients are exactly that. Ingredients for fluids!

You can match either single fluids or a fluid tag (not a list of fluids, though. For that you should create a tag instead). The syntax is exactly the same as vanilla ingredients that match items.

Examples

Matching a fluid

{
  "fluid": "minecraft:lava"
}

Matching a fluid tag

{
  "tag": "c:milk"
},