A pretty common thing in TCG's are "Formats", basically the allowable card pool to use for things like tournaments. The two major divisions are
- Constructed, where you build a deck with your own cards and bring it to the event.
- Limited, where you build a deck from the cards provided by the venue.
It is
really easy to forget that these are the core two because the only real major Limited Events are "Pre-Release" tournaments for each core TCG expansion and they are designed to be very beginner friendly.
Most are familiar with some of the Constructed Formats, and may not even know they are collectively known as such. They include
- Unlimited, where all local language cards are legal.
- Legacy, built out of all cards from the HS-series, Call of Legends, and BW-series (PTCGO exclusive - official events IRL don't use it).
- Expanded, which allows cards from the Black & White Expansion and later (as @Perfect_Shot already said).
- Standard, which allows cards from XY - BREAKthrough and later (again, as Perfect_Shot already told you).
New cards join the Unlimited, Expanded, and Standard Format the third Friday of the month in which they are released. The
oldest cards leave the Standard Format once per year, almost always
after the World Championships. The most common date for "rotation" is September 1st, and we'll usually know
what is going one to three months ahead of it happening. Once they cut zero sets, but it kind of caused problems down the road, which lead to the
most sets ever cut at once (seven, I think). All the other times, it has been three, four, or five sets.
Black Star Promos also rotate, with a certain number serving as the cutoff point.
Most cards enjoy a Standard Format lifespan of about two years.
The
reason for rotation is because it is that or have a massive Ban List. For
years, Pokémon didn't even have a Ban List. In fact, the reason the Standard Format was created (then known as the "Modified Format") was because too many cards from the earliest sets would have to be banned to balance things out back in the day. Instead, the first four sets (one being a reprint set) were all left out of the initial Standard Format...
plus one card was immediately banned. About a year later, another card was banned. The Pokémon TCG officially released in January of 1999. Standard was adopted approximately September of 2001. The set containing
both banned cards left Standard play about September of 2003, and
nothing is banned in Unlimited, so we didn't have
anything banned again until June of 2015. So the game went almost 12 years before the third card banned for Standard Format play became a thing. Since then we've had (I think) three more cards banned, but one of those came
off the banned list.
The
currently banned cards in the Expanded Format are
- Archeops (BW - Noble Victories 67/101; BW - Dark Explorers 110/108)
- Forest of Giant Plants
- Lysandre's Trump Card
Finally, there is a rule that prevents you from running both Professor Juniper and Professor Sycamore in the same deck for Expanded Format play.
Otherwise, even if two cards have the exact same stats and effect
but have different names, you may run both up to the maximum allowed copies (usually four). Some cards have rules text
printed on them that reduces how many you can have in your deck. This isn't an overly common mechanic, however.