Two types of decoys were shown so far. Passive decoys, used to fool active sonar, and active decoys that are much more complex and can be used for a greater range of purposes.
Passive decoys are generally used to fool active sonars of other ships - or, if the situation is really dire - to create a cheap shield to protect the vessel from incoming attacks. Passive decoys can't move much on their own and are as simple as possible. On the other hand, this makes the passive decoy container small enough that not only torpedo tubes, but also missile launchers can fire it.
Active decoys are much more complex than the passive ones. They can move on their own and are generally used to fool passive sonars, as distractions or even as a sort-of escorts when a vessel needs to go close to enemy territory. These decoys are so effective that even a battleship and heavy cruisers couldn't differentiate the decoys to the real ship.Active decoys can also be used to shield the vessel from incoming attacks as well, but there are better uses for them. Control over the active decoys can be passed among the "Fleet of Fog" vessels as well: in the anime Hyuuga, Haruna and Kirishima possessed I-401's decoys and in the manga I-402 and Takao both controlled I-401's decoys.
As was shown in manga by I-401 and Takao, with enough micromanagement, an active decoy can fool humans that are actually inside it into thinking that it is a real vessel. This deceit was discovered only by war veteran Ryoukan Kita when the decoy was brought all the way from Iwo Jima to Yokosuka: the decoy is notably lighter than the real submarine and as such will not have the same effect on the docking arms. It is assumed that the decoy can remain submerged even though it should have positive buoyancy because of graviton floats.
As far as we know, even active decoys don't come with their own torpedoes by default, however, they can be given some after they are activated by the real vessel. In anime, active decoys could also create a net barrier among themselves to capture careless submarines in a tight space.
- While so far only submarines were shown to be able to use decoys, there is no evidence that heavy cruisers or even larger vessels can't use them as well. It is also unknown, whether the vessel must have a potential to maintain a Mental Model in order to be able to use active decoys.
- The ability of U-2501 to create a replica of itself using twenty of her seehunds is similar to active decoys, however, instead of firing an utility torpedo, she will use her drone escorts. Additionally, U-2501's replicas can use weapons, including Corrosive Warhead Torpedoes (if seehunds had any remaining torpedoes on them).