Guest difficulty
For regulations surrounding guest difficulties, see: Ranking Criteria
A guest difficulty, or GD for short, is a difficulty of a beatmap that is not created by the beatmap host. It can usually be identified by its difficulty name containing the guest mapper's username.
Although not mandatory, having guest difficulties can be beneficial in many occasions as they often feature completely different mapping styles between one another, which diversifies the mapset, and helps reduce mapping burnout. Guest difficulties are most commonly requested through private messages between mappers, although they may also be requested by other means such as the in-game #mapping channel or modding queues.
Guest difficulties are not to be confused with difficulties that feature multiple mappers, which are instead known as collabs.
Ownership
An example of the guest difficulty ownership control panel
On the beatmap discussion page, the beatmap host may transfer ownership of guest difficulties to their respective mappers through the Difficulty Owner button. This allows for the guest difficulty owners to resolve pending issues in their difficulties on their own.
Once a beatmap is either Qualified, Ranked, or Loved, ownership of a guest difficulty can no longer be changed except for by members of the GMT or the NAT.