The format changes after the World Championships every year. Rotating out certain sets so that they cannot be used anymore.
Unlimited means any and all Pokemon Trading Cards can be used, except Slowking and Sneasel from Neo Genesis. Unless your TO allows them.
There is also Unlimited RS-On. Which means only cards from the PUSA era, in this case, Ruby/Sapphire on may be used. That is still alot of sets.
Modified means the current format. Which in this case is HP-onwards. This basically means Holon Phantoms, and every set after that, can be used.