Even though low shrinkage has received most of the publicity, low stress at the composite-tooth interface is really the key element. That is the main idea behind these products, which have basically been created just for bases and liners in posterior restorations, although at least one product is now being marketed for Class III and Class V restorations.
Until now, flowable composites have been used for a variety of purposes including being the first increment in the proximal box during the restoration of a Class II preparation. But due to the relatively higher shrinkage of flowable composites compared to their sculptable cousins, we have recommended keeping this layer quite thin. Another reason to limit the thickness of the flowable layer is its light curability. Since the tip of curing light could be 6-8mm away from the gingival wall, curing through a thick layer of flowable composite can be a real challenge. Indeed, in our cure tests of flowables on the gingival wall 6mm deep, it is still prudent to cure for 40 seconds of curing, although 20 seconds may suffice if you use a reasonably powerful curing light.
Lastly, placing a flowable in a thicker layer could involve the contact area, which is subjected to wear. This proximal wear, which presumably is less than that on the occlusal surface, occurs even in virgin teeth and is responsible for the flattening of contacts as we age. That is why the convexities built into various matrix bands may not be appropriate anatomically and physiologically for older patients.
In any event, restoring a contact area with a flowable as opposed to a more heavily filled sculptable composite is somewhat controversial. Therefore, we have always recommended leaving any type of flowable composite or even an ionomer-type base (used in an so-called open sandwich) apical to the contact, even if it is self-cured and the depth of cure with a light is a non-issue.