The proto-bubble (pubble) was a forum post icon used by 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 of the Beatmap Appreciation Team (BAT), who could then place a full bubble icon if the beatmap met their approval. 2go1k
The proto-bubble was retired on the 23rd of April, 2013, with the dissolution of the MAT and the promotion of all current team to the BAT.