Jane, Robyn... it has to do with 'circling' approaches where on an instrument approach you have to reverse course to land on the opposite rwy...I'm trying to find a picture...
• Category A: Speed less than 91 knots.
• Category B: Speed 91 knots or more but less than
121 knots.
• Category C: Speed 121 knots or more but less
than 141 knots.
• Category D: Speed 141 knots or more but less
than 166 knots.
• Category E: Speed 166 knots or more.
http://www.faa.gov/library/manuals/a...ection%20I.pdf
see -page 32 note other pattern are possible those just serve as some examples...most airlines these days either avoid the procedure entirely or limit them to day visual meteorological conditions