The proto-bubble (pubble) was a forum post icon used by members of the Mapping Assistance Team (MAT) in previous iterations of the beatmap ranking process. It was designed to indicate beatmaps believed to be of a suitably high quality for ranking by members of the Beatmap Appreciation Team (BAT), who could then place a full bubble icon if the beatmap met their approval.
The proto-bubble was retired on the 23rd of April, 2013, with the dissolution of the MAT and the promotion of all current team members to the BAT.