Currently, one can only build composite linestyles out of Stroke and Point segments.
We had a client that wanted a linestyle that needed to be a Physical linestyle, but the parallel stroke patterns needed to work like linestyles 1-7: be a fixed dash gap pattern when plotted. While these can be created as stand alone linestyles, these cannot be used in a compound which is then set to Physical as not only do the offsets no longer scale, but the stroke patterns do not scale.
We had to customize the plot driver to make one of the 1-7 linestyles look like their "fixed" dash gap styles and then could create our Physical compound. If we could define a linestyle as non-physical and then assign it as one of the offset linestyle in our composite, we could create a non-plotter dependent linestyle.
As an aside to this, I have seen a number of places where it is recommended to not use linestyles 1-7 as they can be plotter dependent and do not translate well. By allowing the use of non-physical custom linestyles within a physical, compound linestyle, the use of linestyle 1-7 could be eliminated.
The setting of Physical is very obscure in the interface. It should be defined at a lower level in the construction process and more easily seen in the GUI.