The Team Rocket set was the third expansion. As all Pokémon were covered, the cards went on to focus on the evil team from the Pokémon games; Team Rocket. This set introduced different varieties of Pokémon, in this case; Dark Pokémon. This concept allowed you to have a further 4 Pokémon of a particular kind in your deck so you could have 4 Arbok & 4 Dark Arbok in your set. However, only evolved Pokémon were of this kind.
It is also the first set to include a secret card which isn't in the numbering list and the first to contain a card created by Wizards of the Coast; Dark Raichu. It also is the first to include Special Energy cards including Rainbow Energy and various Colorless Energy cards with added effects