A Mixer is a 4-Channel universal additive mixer. It multiplies all inputs by the given percentage value, and adds all outputs together into one. Since the result can be more than 100% the output gets limited to 150%.
If 4 Channels are not enough, Mixers can be cascaded to extend the number of inputs. The Preview only shows the first 3 channels.
Each Channel can be choosen from the list of possible channels. Each channel has a mixing percentage which is the multiplication before the mixing. The Value can be negative, that means that the input is added in reverse.