From Bulbapedia, the community-driven Pokémon encyclopedia.
The 2001-02 Modified format of the Pokémon Trading Card Game (now referred to as Standard format) was the first Standard format used in Wizards of the Coast's DCI sanctioned tournaments, used between from September 1, 2001 to August 2, 2002. 
This format only allowed cards from the Team Rocket through the Legendary Collection expansions and the Wizards Black Star Promos released during this period. There were three exceptions as Ancient Mew, _____'s Pikachu, and Sneasel were banned immediately. This format was informally known as Rocket-On.
The metagame of the Pokémon TCG before the introduction of the Modified Format in 2001 was dominated by decks which consisted mostly of Trainer cards and very little Pokémon cards. Some deck archetypes, such as Haymaker, ran at least 25 Trainer cards, sometimes as many as 40, while only having only between 8 and 12 Pokémon cards. Players during this time disparagingly nicknamed the game "Trainermon", as players felt that this reliance on Trainer cards went against the spirit of the game.
Because Media Factory's philosophy for the game was—at the time—different from Wizards', the solutions that Wizards' usually relied upon to help maintain a viable tournament scene were initially not allowed to be used. For example, Media Factory would not allow Wizards to mass ban cards from organized play like they could with Magic The Gathering, nor could Wizards change a card's game text so that it would be more balanced upon release in the West. Wizards thus had their hands tied to Media Factory's desires for the game, and so Wizards began experimenting with a new tournament format in order to help fix the game.
This new tournament format was called Prop 15 or Prop 15/3, and was used during the 2001 East Coast Super Trainer Showdown qualifiers and during the main ECSTS event on June 23-24, 2001 in Secaucus, NJ. This format did not restrict what sets players could build a deck from, nor would it ban certain cards, like the Modified Format eventually would. Rather, it would restrict players from using more than 15 Trainer cards in their deck, and no more than 3 Trainer cards of the same name. This format ultimately proved to be very unpopular with players, and was also associated with the elimination of the Age 15+ Division during the ECSTS qualifiers and main event, and so it was no longer used after the 2001 ECSTS.
Because of the unpopularity of Prop 15, the Modified Format was announced during a Wizards of the Coast chat on August 10, 2001. Unlike Prop 15, Modified allowed users to use as many Trainer cards as they wanted within the normal rules of the game, but limited players from using certain sets; in this case, starting from Team Rocket on. This gave Wizards the ability to remove broken Trainer cards from sanctioned tournament play without banning them outright. Furthermore, Wizards was also allowed by Media Factory to finally ban a single card, Sneasel, from tournament play. The format itself was based on the "Type 2" Format Wizards used for Magic The Gathering at the time, which only allowed cards from the sets released within the last two years. Magic's "Type 2" Format has since been renamed as "Standard" as well.
The fundamental concepts of the Modified Format—namely rotating out old sets and banning of certain, unbalanced cards—stood the test of time, long after Wizards ceased production of the game, and has long since replaced the Unlimited Format as the main format for sanctioned play, until the introduction of the Expanded format during the 2014-2015 season.
DCI sanctioned tournaments began using the Modified Format during the 2001-2002 season, starting with the qualifiers for the upcoming 2001 West Coast Super Trainer Showdown.
- North America: September 1, 2001 to August 2, 2002
- Europe: September 1, 2001 to August 2, 2002
- Japan: Since the Japanese tournaments were not DCI sanctioned tournaments, Japan did not use this format.