Force (フォース Fōsu) is a term for cards that connected to a Z/X by being placed face-up under that Z/X in Square.
Force is mainly created by the Alter Force ability, but there's also several cards that able to put a card under a Z/X as a Force as well. While the Force itself is located directly under the related Z/X in that Z/X Square, it is treated to be located in the Force Zone under the rule.
Card abilities or effects may refer to a Force possessed by a Z/X. If an ability or effect refers to a Force that was put into a different zone, it refers to that Force's current information in that zone. If it is put into a hidden zone or becomes face-down in that zone, then it refers to the information before the Force was put into that zone instead.
If a Z/X with Force leaves the Square, the Force is put into Trash per Rule Effect.