Flat Zone X has the same basic layout of the two previous Game and Watch stages: walk-off blast lines and no lower blast line. The stage functions similarly to Flat Zone 2 from Brawl, constantly shifting the format of the stage. All of the transformations from Flat Zone 2 remain, with the addition of one more from Melee:
Fire: The starting stage, and the stage that appears when transitioning to another one. Firefighters will be attempting to catch people falling out of the burning building. The levels of the building can be stood on, and the trampoline can be bounced on. Random platforms from Manhole can appear if the firefighters aren't present. The transition is indicated by a firefighter ringing a bell.
Oil Panic: The stage features a multi-tiered gas station that can be stood on. Two customers are present on the ground, and will retaliate against any fighter who attacks them (the man attacks immediately, and the woman will delay one second before attacking). The transition is indicated by a policeman ringing a bell.
Lion: The stage features a three-level cage at the centre of the stage. Two tamers with chairs will move between each level. Touching the handlers inflicts massive damage and enough knockback to push them into the other lion tamer at the other side. The transition is indicated by a lion ringing a bell
Chef: Two sets of platforms appear on the stage, and the chef flips food, dealing damage to any who make contact with him. He can be attacked and sent flying to transition to the next stage quicker. The transition is indicated by a bell-shaped kettle emitting steam.
Helmet: The stage features platforms that disappear and reappear at regular intervals. The stage also features a small house that can be stood on. Stage hazards include an oil spill, making the ground more slick, and tools, which fall from the sky and deal decent damage. The transition is a construction worker ringing a bell.
This is the only flat-based stage in Super Smash Bros. for Wii U where Star KOs are enabled. However, Screen KO's are banned (this was reversed in Super Smash Bros. Ultimate, where neither Star KOs, nor Screen KOs, occur).
Due to its status as a starter stage in Ultimate, Flat Zone X is the only piece of content from the Game & Watch universe to be available from the start in any Smash game.