From BJCP 2008 Style Guidelines:
Introduction to Cider Guidelines (Categories 27 - 28)
The apple and pear varieties are intended to illustrate commonly used examples, not dictate requirements when making the style. In general, adjuncts are prohibited except where specifically allowed in particular styles, and then the entrant must state them. Common processing aids, and enzymes, are generally allowed as long as they are not detectable in the finished cider. Yeast used for cider/perry may be either "natural" (the yeast which occurs on the fruit itself and/or is retained in the milling and pressing equipment) or cultured yeast. Malo-lactic fermentation is allowed, either naturally occurring or with an added ML culture. Enzymes may be used for clarification of the juice prior to fermentation. Malic acid may be added to a low-acid juice to bring acidity up to a level considered safe for avoiding bacterial contamination and off-flavors (typically pH 3.8 or below). Entrant MUST state if malic acid was added.
Sulfites may be added as needed for microbiological control. If used, the maximum accepted safe level for sulfites (200 mg/l) should be strictly observed; moreover, any excess sulfite that is detectable in the finished cider (a "burning match" character) is a serious fault.
BJCP 2008 Style Guidelines - Cider Introduction