Classification was a means of indexing tangible concepts.
The contagious illness encountered by the crew of Enterprise NX-01 in 2153 was partially determined by Doctor Phlox to fall within classification of a virus. (ENT: "Observer Effect") A parasite later discovered by The Doctor that was carried by Tuvok was identified as being "clearly a viral parasite of some kind, but its origin and genome classification are not on record." (VOY: "Flashback")
As of 2369, USS Enterprise-D's holodeck computer contained 5,047 classifications of table in its database. When Deanna Troi asked the computer to show a table, it informed her of the number of tables it had on file and requested that she specify the design parameters. (TNG: "Schisms")