M3U vs Xtream Codes: what's the difference?
Providers usually hand you a playlist in one of two shapes: an M3U link, or an Xtream Codes style login made of a server address, a username and a password. Both get you to the same place, but they behave differently on a TV.
M3U: a text file full of links
An M3U playlist is a plain text file. It starts with #EXTM3U, and every channel is a pair of lines: an #EXTINF line with the name and some attributes (such as tvg-logo for the logo and group-title for the category), followed by the stream's web address.
The upside is simplicity. Almost every player understands it, and you can open the file in a text editor and see exactly what is inside. The downside shows up at scale. One file holds everything, so a big catalog means a big download before you see anything. Movies and series have no real structure, so a series is often just a long run of episodes with names like "Show S01 E03". Guide data comes from a separate file and has to be matched up by an ID.
Xtream Codes style: a small API
The "Xtream Codes" name comes from panel software that was popular with providers; the original software is no longer developed, but the API style it introduced lives on in many panels. With this format you receive a server address, a username and a password. The player then asks the server for exactly what it needs, in structured form: a list of categories, the channels in one category, movie details, a series with its seasons and episodes, and the current and next program for a channel.
That makes browsing quicker, because the player can fetch categories first and details on demand. Posters, ratings and episode lists come with proper structure instead of being guessed from names. The trade-off is that your username and password sit inside the stream addresses, so treat them like any other credential and don't share screenshots of them.
Which one should you use?
If your provider offers both, the Xtream style usually loads faster and gives better movie and series pages. M3U is perfectly fine for a short list, for a one-off channel pack, or when you want to read and edit the file yourself.
Either way, a good TV player caches the list so you don't wait for the whole download every time you open the app.
How Stream2Player handles them
You don't paste anything into Stream2Player. Your provider links your playlist, in either shape, to a numeric activation code, and you type that code on the TV. The app then reads the playlist, builds its categories, and keeps a copy on the TV so the next start is quick. If you'd like to see it in action, follow the setup guide.