Firstly, welcome to world of Pokemon TCG.
Basically, the Tounaments held at every area and shops will most likely have different rulings. The examples are Modified, Unlimited and Limted or Draft for Short.
Modified is a "Section" where the sets that you are able to use follows the ruling decided by POP, or Pokemon Organized Play. Here, a distance between 2 sets, and any sets released in between it can be used. For example, HL-on means Cards from Hidden Legends to the Latest Set Released, which is currently Holon Phantoms. In conclusion, you can use any cards from that specific set chosen, along with other sets released after it.
Unlimited is also a Format where the most broken decks are played and borned. Feraligatr and Lockdown decks Dominated this format. Anyways, all cards ever released can be used here. This excludes some Japanese Cards that aren't and weren't released in the USA in the English Version. Slowking and Sneasel from Neo Genesis were banned when this set was included in the Modified format long ago. ______'s Birthday Pikachu and Anceint Mew were banned as they have effects that only works on Specific days. Rumour has it that Anceint Mew can attack for 130 damage when it is on a New Year but was proven to be false around my place.
Limited or draft is well-known as Pre-releases which are tournaments which gives you the ability to play a deck with the next set's cards 2 weeks earlier. Another one is that when only chosen sets are only able to use. This means only cards from that specific sets can be used. For example, if the set chosen is Delta Species, means all cards in the deck must be from Delta species. This format for the tournament is rarely held at Shops and Areas.
Hope the guide above helps you with your Problem. and no need to Thank Me, all help I can give.