An airsoft scenario is a game played by a large group of players that either resembles a historic battle, or has a fictional element to the back-story of the game. Scenarios usually have things such as certain uniforms for certain teams and weapon allocations, ammo restrictions, and designated roles. Scenarios usually need to last a minimum of 1.5 hrs without stopping. They will usually have 1 or more objectives that need to be acomplished in a certain amount of time, and that may require some time.
The storyline of a scenario usually depicts a battle fought by any powers of the world. They are mostly in the following wars: Civil War, Spanish-American War, WWI, WWII, The Russian Revolution, Korean War, Vietnam, and Iraq War (Sorry if I put any out of order or missed some, please feel free to correct me).
Weapons and ammo
The weapons used most-likely will not be restricted to type, but more to magazine capacity, spare magazines, weapon action, non-loaded ammo, and loading devices. Sometimes, however, weapons must be of the correct time-period to be used.
Most of the time you will be restricted to using the correct uniform, vest, pockets, and camouflage of the time. For instance, if you wanted to use a ghillie suit in a Civil War reenactment, you would need to have a talking-to by a history teacher. But on the other hand, you may be required to have a certain kind of ghillie suit for the Vietnam War.