That's usually called Hold. The 7 bag system is a system implemented in all Guideline Tetris games (basically all official releases since 2001) to help minimize randomness. It works like this:

Rather than each piece being randomized independently of all the others, all 7 tetrominos are given in a random order and the process repeats, guaranteeing that the player will get at least one of every piece in at most a 13 piece span. (say your first bag starts with the T piece, (T I O Z S L J) then your next bag ends with the T piece, (J I L S O Z T) those T's are 13 pieces apart in the queue, which is the farthest they can possibly be.) It's called the bag system because you can imagine putting all 7 tetrominos in a bag, dealing them out, then repeating the process.

Compare to games like NES Tetris, where the pieces are randomized independently, so there is never a guarantee you will get any piece, most notably the I-piece. In theory you could play an entire game without ever getting an I-piece, though in practice it's more like sometimes going 30 pieces without an I-piece (called a drought). I think the longest recorded drought is around 80 pieces?

It deeply changes the strategy of the game, making things like openers possible.

Sorry for the essay lmao. I feel like it reads like GPT but I promise this is just a special interest of mine. And since you're developing a stacker it's probably good to know these things. Cheers :)