And the answer is that the numbers denote the player's place in history: Michael Vaughan's "600", for example, shows that he was the 600th man to represent England in a Test. Separate numbers apply for one-day internationalsone-day internationalsA One Day International (ODI) is a form of limited overs cricket, played between two teams with international status, in which each team faces a fixed number of overs, currently 50, with the game lasting up to 9 hours. The Cricket World Cup, generally held every four years, is played in this format.
› wiki › One_Day_InternationalOne Day International - Wikipedia: in those, Vaughan is No. 161.