Beatmap collaborations, or collabs, are single difficulties or beatmaps with guest difficulties created by multiple people. While the term collab usually refers to beatmaps, it also means any other media created by several participants, such as storyboards or songs.
Beatmap collaborations usually have the word "Collab" or multiple usernames attached to their difficulty name or beatmap description, although it is not a strict ranking criteria requirement. It is also common for frequent collaborators to invent amalgamations of their usernames1 and use them consistently throughout their mapping careers.
The mapping process usually starts with the beatmap host splitting a pre-timed beatmap template into several parts by marking them on the timeline using bookmarks according to the song's musical structure. Relatively long segments, such as verses, are usually split after every 4 or 8 measures. These parts are then assigned to each collaborator according to some agreed-upon distribution, such as by even or odd segments in the case of two mappers.
Mappers complete their parts by laying out the hit objects, applying hitsounds, using inherited timing points where necessary, and then pass their work back to the host, who then combines the changes into a single difficulty by carefully editing its .osu file.
Although mappers usually rely on the parts already completed to make gameplay transitions as smooth as possible, there exist so-called blind collabs such as Mitose Noriko - Unreal Player hosted by Mao, where the mappers cannot see each other's parts until the beatmap is finished. In such cases, hit objects are typically placed at the ends of each section beforehand to indicate where to continue from.
For example, masazi - mayday B, a beatmap by Vass_Bass, has a collaboration difficulty with Reiji-RJ named "Vass-RJ". ↑