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As the indefinite article only sepcifies the number and doesn't refer to a specific something, it just refers to an example- if I say 'a table', then you cannot say which particular table, so I referring to any single example of that item of furniture.
If I say 'a CD', I am just referring to a single CD- not any in particular, so it could be any one of the million available. If I saw 'the CD', it could only be one single CD out of all available (the one I bought yesterday, etc).