The cards, formerly in the Map Room, have been scanned and made available for browsing online so it is now possible to search the card catalogues remotely, without having to come to the reading room. The material contained in them, previously ordered on green slips in the reading room, can now be requested remotely, using the online request form provided.
While it is recognised that this resource falls far short of the comprehensive inclusion of full-level catalogue records for all maps in SOLO, it is hoped that the increased access it provides will be a substantial improvement.
The card catalogue was established a long time ago and some of the terms used are a little old-fashioned.
You may find it useful to consult the the Finding Aids LibGuide for Maps http://ox.libguides.com/finding-maps, for more detailed information on the arrangement of the catalogue and tips on searching.