Lagos was a Sentinel-class landing craft used by the Galactic Empire. After the destruction of the Imperial Communications Center on Lothal, Lagos was used to ferry R4 courier droids to and from the Tellstar in order to maintain Imperial communications.
After the rebel cell known as the Spectres took control of Lothal's Imperial Communications Center, Grand Moff Wilhuff Tarkin ordered the tower destroyed. Without the primary communications tower, the Empire started using R4 courier droids to relay information to place to place, and shuttle Lagos was assigned to carry courier droids to the communications ship Tellstar in orbit around the planet.
When the rebels stole the courier droid 264, they sent their mechanic member C1-10P "Chopper" in its place aboard Lagos in an attempt to gain information about Kanan Jarrus, their leader who had been captured during the raid on the communications tower. Unaware of the real courier's capture, the crew of Lagos delivered Chopper to the Tellstar, which allowed the rebel droid to copy communication logs from it.