The Lasso falloff option provides users a means to draw selection areas directly on a viewport that then limit the application of paint strokes to the surface. The Lasso option is simply a standard falloff type, as documented in the modeling section under Falloffs.
Because of modo's generalized nature, the same Falloff options that are used for geometric edits can be applied to most of the paint tools. In order to utilize falloffs, each paint tool has a 'Use Falloffs' option that must be enabled. Using this option, it is easy to confine paint strokes to specific areas of the model (or viewport). Of course, there are some falloffs that are more useful than others in this regard. For instance, the 'Screen' and 'Airbrush' falloff operators would not be very useful during painting since they have similar attributes to the paintbrushes themselves. Falloffs such as 'Linear', 'Cylinder', 'Radial' and of course 'Lasso' are quite useful when working with the Paint tools.
Typically Falloff operators use the RMB for setup. However, since the paint tools use RMB to set the brush size you must select the Falloff in the Tool Pipe and then use the LMB to define the falloff area before painting. Make sure to select the paint tool prior to painting. Once the Falloff has an initial location you can also easily use the falloff handles to further edit their positions without changing tool selections via the tool pipe.
