The units are different depending whether your GPU supports `GL_OES_standard_derivatives`.  Without derivatives, the unit is a percentage, where 0.0 is no line and 1.0 is all line and no hole.  With derivatives, the units are somewhere in the vicinity of tenths-of-pixels, a unit scale chosen to make available some reasonable default values that work pretty well in both environments.  I'm open to change of course, but I'm going to wait for @pjcozzi to come back from GDC and have a chance to review this.