This
set, for the Arma Hobby 1/72 Ki-43, is a set of Insignia masks. But it is
more than just masks so you can paint the roundels. This set includes the
unit markings along with the common data markings that are provided on the
decal sheet.
These are actually quite easy to use and you have a couple of options for their use. One is that you can paint the background color, apply the mask and then paint the rest of the airframe. Conversely, you can apply these as an outline over the already painted aircraft, though this method will require additional masking to prefent isses with overspray. Or you could do a combination of methods depending on which works best for each application.
Since the set replaces the water slide decals, the mask numbers are the same as the decal numbers to help you with placement.