Tonight's episode of Agents of SHIELD introduced the concept of the multiverse into the series and also made a sly Easter egg reference to the most famous timeline in the Marvel Universe, Earth-616.
Spoilers for Marvel's Agents of SHIELD Season Five, Episode Three, "A Life Spent" Follow.
In tonight's episode, Phil Coulson and his crew continue to try to make sense of how and why they were brought to the future. The only clue they have to go on is the journal of Virgil, the SHIELD devotee who was waiting for Coulson and his team when they arrived.
Coulson studies the journal and comes to the realization that Virgil was taking the trawler ship out into the ruins of Earth and was performing some kind of recon. He had the different areas of what's left of the planet marked out in different sections and one section, 616, was visited frequently.
That numerical designation is significant because, in the Marvel Comics multiverse, Earth-616 is the designation of the universe where the majority of stories told in Marvel Comics series take place. This is the original timeline that appeared in Fantastic Four #1 in 1961. The term was first used in the Captain Britain story "Rough Justice" by Alan Moore and Alan Davis, which was published in the Marvel UK anthology series The Daredevils in 1983.
Marvel Comics has tried to distance itself from that numerical designation, for whatever reason, preferring to refer to their universe as simply the Marvel Universe or, if clarification is necessary, the Prime Marvel Universe. However, the 616 label has been latched onto by fandom and they continue to use it in conversation and fan culture.
This isn't the first time that Marvel's Agents of SHIELD has used 616 as an Easter egg for Marvel fans. The Bus, the original flying airbase of Phil Coulson's team, was designated "SHIELD 616."
Perhaps Virgil was specifically instructed to label that hunk of rock 616 in the hopes that it would remind Coulson's team of the bus. That rock is where the signal from Earth was bouncing off of, and whoever is still down on the planet seems to have been expected Coulson's "delegation."
Marvel's Agents of SHIELD airs Fridays at 9 p.m. ET on ABC.