An article from 2010: For Baseball Archivists, a Tag Ends Every Play, by John Branch at the New York Times
“Your archive is only as good as what you know is in it,” said Elizabeth Scott, M.L.B. Productions’ vice president for programming and business affairs.
It is not only the game action that is tagged. If a squirrel runs onto the field, the play will be tagged with “animal.” If there is a shot of a man sipping a beer, there is a “drinking” option under the “fans” category. Those frequent shots of boats bobbing in San Francisco Bay during Giants games are tagged with “boat.” “Pine tar,” “rosin,” “babies,” “bat boys/girls” and “on-deck circle” have their own notations. So do “fist pump,” “curtain call” and “throw equipment.”