From my experience, subshrubs commonly have woody tissue near the base of the plant. One local example is Gutierrezia sarothrae which is herbaceous about half its height. In the winter, the herbaceous tissue mostly dies back and what is left becomes woody. In the spring the cycle starts over again.
In my "Woody Plants of Utah" book, a subshrub is defined as "a perennial plant, woody only at the base or slightly above." Many plants can be both shrubs and subshrubs depending on their conditions but a lot of them never cross into the shrub threshhold.
As someone who has worked at a garden center, one problem I have seen is that people try to divide their plants the same way they would an herbaceous perennial.
