Difference between Feed Additive and Feed Supplement

Can anyone give me information about the difference between Feed Additives and Feed Supplements? As far as my knowledge goes Feed Supplements are those added to cover up the deficiencies of items already present in the feeds and the Additives are those which are added to the feeds which are absent in them in order to make it more effective or palatable or for better flavour. Am I correct? Are there any case laws in support of this issue?