The Weight Map Mixer Op property is created when you connect extra weight maps to a single parameter. To do this, right-click on the connection icon to the right of the parameter's slider in the corresponding property page then choose Add, pick a weight map, and click outside the pop-up explorer.

To redisplay: Expand the Mapping Node of the cluster in the explorer and click a Weight Map Mixer Op node. Note that this is a feature intended for advanced users.

There is one weight-map mixer for each weight map that is blended.