If you add a particle system to a mesh many aspects of this particle system can be controlled by the weights in vertex groups. This includes the density of the particles and the length of hair. These options can be found in the Vertex Groups section of the Particle system tab.

To restrict for example the distribution of particles on a mesh to vertices visible from the active camera you would create a vertex group with WeightLifters Camera mode and then enter the name of that vertex group in the Density slot of the vertex groups section.

No, the Attribute node in Cycles can only refer to uv maps and vertex color maps but WeightLifter can just as easily create a vertex color map. See also the next question.

To use a vertex color layer after you have created one you can add an Attribute node in the node editor (Add → Input → Attribute) and enter the name of the vertex color layer in the text box. The color output socket can then provide the colors, which you can manipulate as needed.

WeightLifter will store the values it calculates into the active vertex group or vertex color layer (and create a new group or layer if there is none yet). To use more than one layer you have to create a new one (by clicking the + sign next to the list of vertex groups or vertex color layers) and give it an appropriate name.

Then click the layer or group you want to change (it will be highlighted) and then invoke WeightLifter.

Make sure you have enough subdivisions in the objects that you create a weight map for. Weights and vertex colors are assigned per vertex so if there are not enough vertices the weight map will look coarse.

As of version 20150222 of WeightLifter particle systems are notified automatically to recalculate if they are using the vertex group. If you are using an older version you may use the remedy below.

Blender doesn’t update the particle distribution visible in the 3D view when you change the vertex group that drives the density. The easiest way to update the view is to click the ←→ button to the right of the density group selector in the vertex groups section of the particles tab twice.

If you tweak the curve of the weight edit modifier the weights displayed in weight paint mode do change interactively but unfortunately a particle system that uses this vertex group isn’t updated immediately: you have to apply the modifier and then tweak the density group as discussed in the previous question.

If a weight map that drives for example the density of a particle system is changed, the system is recalculated and the positions of all the particles will change. This is inherent in the way Blender works. An option is to combine all the individually baked maps into a single map with the Combine Bakes button (in the Scene options). This will probably not restrict the particle system just as neat to the camera as calculating this for each individual frame but might still save you a lot of particles depending on your scene and these particles will not move because of frame changes.