Your question applies to the Ashes 2009 test matches:
The number is located on the shirt beneath the England or Australian emblem, of every player. The number is allocated for each player that represents his country, earning his cap in the olden days. These numbers date back to 1876 where Tom Armitage from England received the first cap number for England. The test match was England vs. Australia.
Every country has his own set of cap numbers, which is allocated to their players that have represented them in international cricket.
The player keeps this number for eternity, as it was allocated to his name.
One Day International (ODI) the 50 over limited version of cricket was introduced in the 1960's and there a different set of numbers for these players that represented their country in ODI's.
The third and newest version of the game Twenty20 (Pro20) where the over’s are also limited have a different set of numbers for this version of cricket.
JUST WATCH OUT!
The number on their cap and shirt are their player cap represent number, the number on their back in ODI's and Pro20's are their squad number and this is not fixed and can chance every year at the start of a new season, this is for the players own choice of what number he would like to be. This number does not indicate a position or type of player. Most players choose their bating lineup number or even a lucky number.
PS:
South African cricket was isolated from the rest of the world for 21 years - from 1970 to 1991 - because of the apartheid policies of the former government. Isolation came at a time when the country could fairly claim to be the top test-playing nation in the world after the Springboks (now Protea's) had pummeled the powerful Australians 4-0 with ease.
Because of the policies of its government, however, South Africa had only ever played against Australia, England and New Zealand. When the country was readmitted to world cricket in 1991 it was able to face all opponents, and India was the first to welcome the 'new' South Africa into the international game.
And I think our number system was recognized by the South African public and sporting government in 1991 because of our apartheid history.
Hope this is of help, and enjoy the Ashes.
Cricket supporter from SOUTH AFRICA