Outflow is a material that deletes other materials. By default it is black Wall. Liquid Outflow is possible. Outflow does not delete other materials it is mixed with, for example QO (Water + Outflow) will not delete Water but will delete anything else.
Outflow's selective deletion is actually extremely useful for acting as a "filter" for cleaning things. If you have "dirty water" you can make Water + Outflow that ignores the Water but deletes anything else mixed in, leaving clean water. Outflow is useful any time you want to have things automatically removed from your sim without needing to send it off the side of the canvas. Outflow is extremely useful for controlling the overpowering effects of Inflow bombs. Sadly the aggressiveness of Outflow can not be easily controlled, so special care and design must be practiced when using Outflow. Outflow causes lag in the game whenever it is eating something. Outflow must be in contact with a particle to destroy it, meaning it attacks dense materials more slowly.
Outflow also has "remote force" capabilities. When combined with Jet, Outflow's ability to delete can be extended some distance away from the actual Outflow particles when subjected to extremely high forces. This could allow the creation of destructive forcefields.