Passage: Xiao'erjing can be divided into two sets, the "Mosque system", and the "Daily system". The "Mosque system" is the system used by pupils and imams in mosques and madrasahs. It contains much Arabic and Persian religious lexicon, and no usage of Chinese characters. This system is relatively standardised, and could be considered a true writing system. The "Daily system" is the system used by the less educated for letters and correspondences on a personal level. Often simple Chinese characters are mixed in with the Arabic script, mostly discussing non-religious matters, and therewith relatively little Arabic and Persian loans. This practice can differ drastically from person to person. The system would be devised by the writer himself, with one's own understanding of the Arabic and Persian alphabets, mapped accordingly to one's own dialectal pronunciation. Often, only the letter's sender and the letter's receiver can understand completely what is written, while being very difficult for others to read. Unlike Hui Muslims in other areas of China, Muslims of the northwest provinces of Shaanxi and Gansu had no knowledge of the Han Kitab or Classical Chinese, they used Xiao'erjing. Xiao'erjing was used to annotate in Chinese, foreign language Islamic documents in languages like Persian.Xiaojing was used mostly by Muslims who could not read Chinese characters. It was imperfect due to various factors. The differing Chinese dialects would require multiple different depictions with Xiaojing. Xiaojing cannot display the tones present in Chinese, syllable endings are indistinguishable, i.e. xi'an and xian. Xiao'erjing was much simpler than Chinese characters for representing Chinese.

Question: Understanding of what alphabets was needed by the writer?
Arabic and Persian