FYI - I wanted to learn what all the parameters in Physics paint do, so I asked. It gave me instructions on how to use it that are not correct.
It says select the object you want to paint but that is not correct. It wants you to select the objects to paint onto a surface when you activate it. If you don't have those objects selected it tells you you need to select those objects before activating the tool. Also, if you look at the tool options they do not match the tool description.
I feel like there are 2 tools that use the name Physics paint. One that drops objects intoa scene and maybe somewhere else a tool for adding physics to objects?