The Prop Polygon Tool allows props to be placed within arbitrarily shaped regions.
Prop polygons are limited to producing 10,000 individual prop instances each.
See Polygon Editing.
Alternatively, click and drag a prop asset from the Library Browser directly onto the desired prop polygon. (You do not need to select the Prop Polygon Tool first to perform this operation, because it works from any tool.) Once done, RoadRunner automatically enters the Prop Polygon Tool and selects the changed prop polygon for further editing.
Click the Prop Polygon Tool button.
Click the prop polygon you want to change.
Click the Bake button in the Attributes Panel. This operation turns all instances of a prop within the prop polygon into individual instances. This operation also deletes the existing prop polygon and automatically enters the Prop Point Tool.