"A small and intense map that takes place in a derelict UNSA ship that is teetering at the edge of a black hole."
— In-game description
Mayday is a multiplayer map featured in Call of Duty: Infinite Warfare. It is set on the UNSA Frigate Icarus 03, a UNSA ship made by the U.S., Mexico, Japan and the U.K. on the verge of falling into a nearby black hole, which is refered to as Cygnus X-3.
Currently there are few details about the specific ship class that the Icarus 03 belonged to. However, it's implied that in general UNSA/SATO used Frigates in peacetime primarily for survey duties (with a touch of SIGINT on the side) and deep space exploration. In wartime ISR taskings such as Deep Range reconnaissance would have been seen as their primary role. Secondary roles included picket duty and acting as anti-fighter/anti-missile escorts for convoys, task forces and the like.
While it is not clear how many Frigates were in UNSA service prior to the war, it is known that only a few were in-system when the war brokeout, in large part due to the UNSA civilian leadership's refusal to allow Admiral Raines to put SATO forces on a war footing or even on heightened alert in the aftermath of the Europa Incident, in a fatally misguided attempt to avoid raising tensions between the UNSA and the SDF even further.