Vertex Painter lets you easily create weight paint (vertex grups) and vertex paint (vertex colors) with three different masks called "Camera", "Slope" and "Altitude". They can all be either combined or used separately with changes made in real time.

Camera:

Camera mode is what it sounds like, creating paint only within your active camera. This allows you to reduce the amount of unneccessary CPU power while working in your project and only have the for example particles in weight paint mode where they are seen.

Slope:

Slope mode creates paint determined on the slope of the mesh. For example, you seldom want to have trees growing on a cliff edge but ususually where it is plain ground. Or perhaps to have a rock material where the slope angle is highest and grass, sand etc. where it is more plain. The settings allow to set the slope thresholds exactly as you like with a few clicks, also with the opportunity to invert the paint by shifting the two threshold parameters.

Altitude:

In altitude mode, you can set the paint depending on the height. It allows you for example to only grow grass/trees above the waterline and under the treeline, or perhaps be used to have a snow material on the top of mountains.