The Exemplars are a legal superhero team based in Boston and dedicated to upholding the public good. Most of the Exemplars were members of the previous Exemplars team (based at San Francisco) who were sent to a far distant galaxy by a Skrull superweapon in 2003 and who returned to Earth in 2008 to discover that the rest of the team had disbanded as a result of the tumultuous Skrull Secret Invasion of 2006. The returning Exemplars relocated to Boston at the suggestion of their teammate Songbird II, as the local superteam where she was going to college (the Bay Batallion) had disbanded following the repeal of the Superhuman Registration Act in 2007 and the local authorities were keen to find replacement superheroes to take their place.
The Exemplars were originally formed through the merger of the San Francisco X-Men and the Los Angeles Champions, and while in San Francisco had publicly supported the Xavier Vision (at its simplest to work towards the peaceful, integrated co-existence of mutants and non-mutants on this world), and to oppose terrorism inspired by genetic differences. On their return from space, the current Exemplars discovered that the events of the five years that they had missed while in space had resulted in a radical realignment of public opinion in favour of superpowered humans of any kind, including mutants, as part of humanity's defences against the "alien threat". Whether the general acceptance of the Xavier Vision by the public will persist if the "alien threat" abates is yet to be seen.
In general terms, the campaign setting is a divergent version of the Marvel Universe in which so much game time has passed that a new generation of heroes has largely replaced Marvel's iconic heroes. The campaign has now been running for 28 years, and 24 years have passed in the campaign world. Spider-Man and the Human Torch are both in their late forties, the original Captain America is Director Steve Rogers of SHIELD, and Franklin Richards is in his early thirties and an active member of the Fantastic Family. The Original X-Men are all in their mid forties except Jean Grey, who was brought forward eight years in time (from 1982 to 1990) by Cable instead of dying on the moon at the end of the Dark Phoenix Saga (Uncanny X-Men #138, published October 1980).
The campaign departed from the comics in Uncanny X-Men #180 (published April 1984): the odd machine in Central Park was a trap set by Arcade for the X-Men, and not the Beyonder's portal to take the X-Men the Secret Wars. Anything that happened to the Marvel Universe in a comic published before or at the same time as Uncanny X-Men #180 definitely occurred in the campaign world, though not necessarily exactly as presented: for example, in the campaign world the Fantastic Four gained their powers testing an experimental spaceship for a trip to Mars, not the Moon. Anything published in later comics may or may not have happened, and may have happened differently if it did: for example, the history and location of Genosha are different in the campaign world to the published comics, as it is a "post-human" society created when mutants occupied the island of Diego Garcia on 22 November 1994 and currently ruled by its elected president, Magneto.
The campaign world also includes many characters from other superhero settings, including DC Comics (for example, Captain Atom), Cyclone Comics (for example, the Southern Squadron), and the Champions Setting (for example, Eurostar). Anything published about these "imports" probably didn't happen, or happened differently if it did: for example, Captain Atom still received his powers because of a US military test involving an experimental nuclear weapon in the late 1960s, but the scientists involved in the test included a youthful (pre-Hulk) Bruce Banner.
The modern age of heroes (beginning with Fantastic Four #1, published August 1961) started in 1976. The "all new, all different X-Men" (Giant-Sized X-Men #1, published May 1975) were formed to rescue the Original X-Men from Krakoa in 1980. The compressed timeline for the modern age of heroes means the events of comics published from 1961 to 1975 took place from 1976 to 1979, while the events of comics published from 1975 to 1984 took place from 1980 to 1983.