It can be used as both but that does not make it accurately used as both. Actually, "dB" does imply a relative scale. It is often confused with dBm which is an absolute power measurement (dB gain in relation to a fixed value of 1 milliwatt). A good summary of dBm is provided at: http://www.physlink.com/Education/AskExperts/ae350.cfm

This subject always seems to cause a little confusion...

-Cory