Inflow is a "spout" as such. Although the default material is set to Water, when mixed with another element it becomes a source for it. Inflow particles can be a liquid, a solid, or a wall.
The only elements that cannot be inflowed are Rigid, Wall, Delete, and Users. Inflow combined with Users will allow the source particles to be moved. A liquid form of Axis can be inflowed. There can be an Inflow for Outflow, too.
Inflow only emits liquid when it touches something. Another particle of any material will work, but a single point of Inflow does nothing. Inflow can be destroyed when combined with Powder or Fuel.
Pure Inflow (made in Mix Mode by pressing esc+I+esc) behaves like a liquid, and when shaken or hit hard enough it will create a very large amount of liquid almost like a bomb. Pure Inflow generates material "N", Null liquid.