In Rainbow Six there are different Operators from all over the world, each with their own special abilities. Rainbow Six is a team built from the best special forces in the world, it's a mix of different nations.
The operator system gives options for what people like and what suits the team best. The operator will reflect the player's playstyle. Once that operator is picked by a player it is removed from the pool.
In addition to these operators, there is also the option to choose the Recruit operator, recruits are versatile operators, they have no special ability/gadget, but they have a wide range of weapons/standard gadgets to choose from depending on whether they are attackers or defenders.