The Cup has a level restriction on it as well, only allowing Pokémon between the levels of 50 and 55 to compete. In addition, the three-Pokémon team chosen by the player at the beginning of a battle may not have a total level sum above 155. This means as well that the six-Pokémon team chosen to compete can't have more than four Pokémon of level 55.
There are four levels that may be challenged; each level up must be unlocked by defeating the eight Trainers on the previous level.
The Poké Cup that appears in Pokémon Yellow and Pokémon Stadium is based on the Nintendo Cup '97, a real life event that took place in Japan that year.
The Poké Cup that appears in Pokémon Stadium 2 is also based on another real life event: the Nintendo Cup 2000.