As the title really. You can buy type A or type B drain off cocks for central heating, etc. What's the difference?
http://www.toolstation.com/shop/Plumbing/Valves/MT+Cock+Type+A+15mm+Plain+Shank/d20/sd2696/p71396
http://www.toolstation.com/shop/Plumbing/Valves/MT+Cock+Type+B+15mm+Plain+Shank/d20/sd2696/p70876
http://www.toolstation.com/shop/Plumbing/Valves/MT+Cock+Type+A+15mm+Plain+Shank/d20/sd2696/p71396
http://www.toolstation.com/shop/Plumbing/Valves/MT+Cock+Type+B+15mm+Plain+Shank/d20/sd2696/p70876