%SKIN% expands the skin search path. For instance, SKIN can be set to catskin, bearskin.
The SKIN setting can be overridden using the URL parameter skin, such as ?skin=catskin,bearskin
You can also extend the existing skin path using covers - see COVER
%SKIN% expands to jaco, public.jaco.nat, jaco.nat, natedit, nat