Context: Data compression can be viewed as a special case of data differencing: Data differencing consists of producing a difference given a source and a target, with patching producing a target given a source and a difference, while data compression consists of producing a compressed file given a target, and decompression consists of producing a target given only a compressed file. Thus, one can consider data compression as data differencing with empty source data, the compressed file corresponding to a "difference from nothing." This is the same as considering absolute entropy (corresponding to data compression) as a special case of relative entropy (corresponding to data differencing) with no initial data.

Question: What can be looked at as a special case of data differencing? 
Answer: Data compression

Question: What is made up of producing a difference given a source and a target?
Answer: Data differencing

Question: What can classified as data differencing with empty source data?
Answer: data compression

Question: What can be looked at as a special case of relative entropy?
Answer: Unanswerable

Question: What is made up of producing a compressed file given a source and a target?
Answer: Unanswerable

Question: What can be classified as data differencing with corresponding data?
Answer: Unanswerable

Question: What means producing a target given only a special case?
Answer: Unanswerable

Question: What means producing a compressed file given no initial data?
Answer: Unanswerable