This is a unit of storage capacity or rate of transmission, as in "our highspeed link could transfer 3 library of congresses (sic) in 3.5 seconds".
There appears to be no agreement on whether the unit represents library of congress as
- ASCII text files (which would be lossy, since there are many non-ASCII books and many with images)
- page images (at some unknown level of resolution)
- the metadata associated with the collections
- the library of congress website.
Generally 1 or 3 is used by vendors and reporters, since it results in bigger throughput.