From the Super Mario Wiki
The Checkpoint Flag (also known as the Midway Flag or Halfway Flag) is a recurring object that signifies a checkpoint in several Mario games. They are similar to the Midway Gates from Super Mario World. If Mario loses a life after touching a Checkpoint Flag, he restarts where the Checkpoint Flag is. If Mario is in his small form, the flag almost always restores him to Super Mario. When the player touches a Checkpoint Flag, they will be rewarded with 2000 points in some games.
Super Mario series
New Super Mario Bros.
Checkpoint Flags first appear in New Super Mario Bros. as white flags with a picture of a Mushroom on them, though not all courses have them. They're invisible, only appearing when Mario reaches a checkpoint zone (similar to Super Mario Bros.). If Mario loses a life, he will restart the course from the flag. However, if the player exits the course, plays another, then returns to said course, they will have to start from the beginning.
New Super Mario Bros. Wii
Checkpoint Flags reappear in New Super Mario Bros. Wii. In this game, they appear as black flags with Bowser's symbol on them. The character who touches one changes the flag to his flag (Mario and Luigi's flags have the same colors and symbols as their caps (red with an "M" for Mario, green with an "L" for Luigi, and the Toads' flags become their colors with their heads), and he gains 2000 points. Additionally, if Mario is currently Small Mario, he becomes Super Mario when the Checkpoint Flag is activated, similar to the checkpoints in Super Mario World.
As a side note, if Luigi or one of the Toads were to activate a Checkpoint Flag, everybody were to lose a life, and the character who touched the flag drops out of the multiplayer session, the players must start over from the beginning of the level.
Super Mario Galaxy 2
Checkpoint Flags reappear in Super Mario Galaxy 2, merely saving one's progress in the mission (due to the game having a health meter and no score system). Depending on which galaxy/mission the player is in, there may be anywhere from one to several Checkpoint Flags positioned between the beginning and the end of the level. Some missions, such as The Perfect Run, may not feature any Checkpoint Flags at all. In addition to having Checkpoint Flags activated by Mario or Luigi themselves walking into them, they can also be activated by the Co-Star Luma or by Yoshi's tongue.
Super Mario 3D Land
Checkpoint Flags reappear in the Nintendo 3DS game Super Mario 3D Land, acting similar to how they did in New Super Mario Bros. Wii (as the game lacks a score system), and like before, some stages like World 2-1 may not have Checkpoint Flags at all.
New Super Mario Bros. 2
Checkpoint Flags reappear in New Super Mario Bros. 2, acting as they do in New Super Mario Bros. Wii. In Coin Rush, touching the flag does not give points, but adds 50 or 100 seconds to the time limit. They also do not appear in every level of the game, like in Rainbow and Warp Cannon Courses.
New Super Mario Bros. U
Checkpoint Flags appear in the Wii U game, New Super Mario Bros. U acting as they did in prior 2D games. The flag is wrapped around the pole this time.
Super Mario 3D World
Checkpoint Flags reappear in Super Mario 3D World, looking and acting as they did in New Super Mario Bros. Wii. As in home console New Super Mario Bros. games, the color of them corresponds to the character who touched it, as well as their emblem, which means Mario and Luigi retain their flags, Peach gets a pink flag with her crown, Toad has a blue flag with his face, and Rosalina's cyan-colored flag shows a Grand Star.
Super Mario Maker
Checkpoint Flags are included in Super Mario Maker in an update released on November 4, 2015. They can be placed in a course by shaking an arrow sign, and like an arrow, can be rotated. If a Checkpoint Flag is placed in a level, then in addition to clearing the course normally, players must also be able to clear the course from the checkpoint. Two Checkpoint Flags can be placed per stage (one in the main area of the course and one in a sub-area). They also appear slightly smaller than in New Super Mario Bros. U. The Checkpoint Flag appears in Super Mario World instead of the Midway Gate, although the Giant Gate still is there.
Super Mario Run
Checkpoint Flags appear in Super Mario Run in Toad Rally mode, having the same design as in the New Super Mario Bros. games. They are placed roughly halfway through the level, and whichever of the two players passes by the flag first earns 10 coins. Once one player activates it, it has no effect on the other player if they pass by it themselves. If the player activates it first, the flag will change into the current character's respective emblem. If the opponent activates it, the flag will turn gray with Bowser's emblem still on it. Checkpoint Flags do not turn Mario into Super Mario if he is small.
Super Mario Odyssey
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