Deciphering Zubulake

The basic outline established by the famous decision in Zubulake v. UBS Warburg, provided that a producing party must bear the cost of producing online data, near-line data (such as that on optical discs), and data from offline storage, such as that on magnetic tapes. The requesting partry only pays if data is recovered from back-up tapes or fragmented data in file slack. But what's the difference between magnetic tapes and back-up tapes? According to Zubulake IV, tapes are accessible when they are used for regularly for information retrieval, but only fall in the back-up category when they are maintained strictly for disaster recovery. There is an exception if a company can locate documents for a particular employee on the tapes used for disaster recovery - then the data is deemed accessible and cost does not shift.