Here is a quick cheat sheet for all the Angelus Paint Additives:
Duller - Is for dulling the paint to make it appear more flat or matte
2-Thin - For thinning the paints if you need them thinner for an airbrush or another thinned out application.
2-Hard - For hardening the paint for projects that do not bend or flex. Usually we suggest this for non moving parts like glass or plastics. This changes the flexibility of our paint to make it more ridged and scuff protectant.
2-Soft - For making the paint more launderable and soft. This is great if you are doing sock liners, laces, or any other canvas, denim, or porous materials. Make sure to heat set with a heat gun to make the paint not stay liquid.