Ilford Ilfostop is a low odor acid stop bath. Normally, when development is completed, films and papers are subjected to a stop bath rinse. This immediately halts the developing process by neutralizing the developer.
Following this, the film or print is exposed to fixer (hypo). The advantage of the stop bath process is that it extends the life of the fixer. Some people do not use stop bath and, in that case, you must use an inordinate amount of fixer to erase the effects of the developer. If a stop bath is not available or chosen, the film or print should be thoroughly soaked in a water bath or rinse - with frequent changing of the water supply. Stop bath gives you the assurance that your developing stage is completely "over" and that none of the developer will be carried over to the fixer.
Ilfostop contains an indicator dye that is pH sensitive and changes color from yellow to purple as the stop bath becomes exhausted. It is specifically recommended for dish and tray processing of paper or deep tank processing of film.