Hi I will be happy to answer your question. SDR is a flowable composite liner that can be light cured up to 4mm in depth. It is usually used as a bulk fill dentin replacement and a regular( a harder) capping composite is placed on top. Flowable liners usually do not respond well to heavy wear areas and that is the reason a harder hybrid type composite is usually placed on top. That being said, if the cavity being filled is very small, or a groove of the tooth, or possibly just a root surface ares that is small, or a baby tooth, one might use only the SDR. hope this helps.
http://www.dentsply.co.uk/Products/Restorative/Composites/SDR.aspx Here is a link to the Dentsply website for further information.