Help:Surveyor Topology Advanced Displacement Map Area Select
Click on Select Area (B). Click LMB and drag the mouse to make a large square as shown below. The size of the square (or rectangle) determines the area that will be affected by the next operation.
TRS2019 Effects Layers (Clutter and TurfFX)
When clutter effect is selected, the Select Area tool will allow you to select an area, to then use the Fill Area (F) tool to 'fill' that area with the selected effect.
See Effect Layers for further information on creating, using, and editing Effect Layers.